Why does it have black screen with 'enter password' prompt?

You can try the following:

If your S.O. is Windows you can try any Windows Password Recovery Tool

Alternatively you can ty( if Windows):

Step 1: Boot or Reboot your Windows laptop (it is a Windows 7 example ).
Step 2: Hold on F8 to enter Advanced Boot Options before the Windows 7 loading screen appears.
Or Press CTRL to enter Advanced Boot Options when computer power on.
Step 3: Choose Safe Mode with Command Prompt in the coming screen, and press Enter.
There are three options for choice, Safe Mode, Safe Mode with Networking and Safe Mode with Command Prompt. In order to reset Windows user password with command prompt, select Safe Mode with Command Prompt.
Step 4: Wait for Windows 7 to load files until Windows login screen comes out.
Step 5: Log in with an administrator account or built-in administrator when you see login screen.
Step 6: Command Prompt runs automatically. Type net user in the Command Prompt and press Enter to see how many user accounts in locked laptop. Then all of laptop users will be listed in the Command Prompt.
Step 7: Choose one administrator to reset forgotten password for laptop.
Type net user fuzi 123 and hit Enter. Suppose 'fuzi' is your locked Windows user name. Then '123' would be new password you reset for it.
Step 8: Login your laptop with the new password after successfully laptop password reset.

this is normal for mysql server. It's simply wanting you to set the administrator password. the black box appears, you type your password, the black box closes.
This means the password is now set to what you typed in the black box (command prompt box)

How can i unlock my local disk C:

Which operating system ?
Restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer
Windows Advanced Options Menu

Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode

Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu

Use the up and down arrow keys to move the highlight to your choice
select safe mode with command prompt

Click start run type cmd press Enter
black screen should open DOS mode
this will vary depending on your operating system

type in net user username
For eg. If you want to change administrator password , type net user Administrator

It will prompt you to enter the new password.
Enter the new password and the password is changed

or

Click "Start," then select "Run." In the text box displayed, enter "control userpasswords2" and press "Enter." This opens a configuration window.

Click the user for whom you want to disable the password login.
After selecting the user, remove the check mark labeled "Users must enter a username and password to use this computer."

Click "Apply."

A dialog box will prompt you for the password.
Enter the user's password to authorize the changes and press "OK."

Reboot your Windows XP computer.
When it shows the list of user profiles, click the user whose password was disabled.

You should be able to log in without a password.

Restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer
Windows Advanced Options Menu

Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode

Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu

Use the up and down arrow keys to move the highlight to your choice
select safe mode with command prompt

Click start run type cmd press Enter
black screen should open DOS mode
this will vary depending on your operating system

type in net user username
For eg. If you want to change administrator password , type net user Administrator

It will prompt you to enter the new password.
Enter the new password and the password is changed

or

Click "Start," then select "Run." In the text box displayed, enter "control userpasswords2" and press "Enter." This opens a configuration window.

Click the user for whom you want to disable the password login.
After selecting the user, remove the check mark labeled "Users must enter a username and password to use this computer."

Click "Apply."

A dialog box will prompt you for the password.
Enter the user's password to authorize the changes and press "OK."

Reboot your Windows XP computer.
When it shows the list of user profiles, click the user whose password was disabled.

You should be able to log in without a password.
also

Click the "Start" button.
Double-click "User Accounts."
Select the user account you wish to change.
Select "Change My Password."

If you wish to remove the password, select "Remove My Password."
also

Click start control panel Open User Accounts in Control Panel. On the Users tab, click the name of the user whose password you want to change, and then click Reset Password.

Just click In New password and Confirm new password, type the new password by just clicking, and then click OK.

Notes You can change passwords only for local computer accounts, such as guest, administrator, or accounts you create for your computer.

You cannot change your network logon password in User Accounts.
To change your own password, press CTRL ALT DELETE and then click Change Password

Which operating system ????.....
Restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer
Windows Advanced Options Menu
Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu
Use the up and down arrow keys to move the highlight to your choice
select safe mode with command prompt
Click start run type cmd press Enter
black screen should open DOS mode
this will vary depending on your operating system
type in net user username
For eg. If you want to change administrator password , type net user Administrator
It will prompt you to enter the new password.
Enter the new password and the password is changed
http://wiki.ljackson.us/Control_Userpasswords2
if this fails you will need to
Select safe mode with networking to download
you could download all of these or just one save them to a usb drive for later use if one fails to remove the password
Windows Password Recovery Basic
http://majorgeeks.com/CmosPwd_d239.html
A CMOS BIOS password recovery tool
Editors note: This tool should only be used by experienced computer users.

Restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer
Windows Advanced Options Menu
Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu
Use the up and down arrow keys to move the highlight to your choice
select safe mode with command prompt
Click start run type cmd press Enter
black screen should open DOS mode
this will vary depending on your operating system
Click "Start" icon in the lower left corner of your computer screen
Type the word "run" in the window that appears.
A second pop-up window will appear; type in the letters "cmd," then select "OK."
This will bring up the black DOS command screen.
Type next to the administrator account the following command: net(space)user(space) the username(i.e. johnadams) *. After typing this command, hit "ENTER" on your keyboard.
Type in a new password in the command screen prompt that ask you to type a password. Type the new password again once you are asked to confirm.
Type the word "Exit." This will bring you back to your login screen.
Enter the new password that you selected.

Restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer
Windows Advanced Options Menu
Please select an option:
Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode
Start Windows Normally
Reboot
Return to OS Choices Menu
Use the up and down arrow keys to move the highlight to your choice
select safe mode with command prompt
Click start run type cmd press Enter
black screen should open DOS mode
this will vary depending on your operating system
type in net user username
For eg. If you want to change administrator password , type net user Administrator
It will prompt you to enter the new password.
Enter the new password and the password is changed
or
Click "Start," then select "Run." In the text box displayed, enter "control userpasswords2" and press "Enter." This opens a configuration window.
Click the user for whom you want to disable the password login.
After selecting the user, remove the check mark labeled "Users must enter a username and password to use this computer."

Click "Apply."
A dialog box will prompt you for the password.
Enter the user's password to authorize the changes and press "OK."
Reboot your Windows XP computer. When it shows the list of user profiles, click the user whose password was disabled.
You should be able to log in without a password.
also
Click the "Start" button.
Double-click "User Accounts."
Select the user account you wish to change.
Select "Change My Password." If you wish to remove the password, select "Remove My Password."
also
Click start control panel Open User Accounts in Control Panel. On the Users tab, click the name of the user whose password you want to change, and then click Reset Password.
Just click In New password and Confirm new password, type the new password by just clicking, and then click OK. Notes You can change passwords only for local computer accounts, such as guest, administrator, or accounts you create for your computer.
You cannot change your network logon password in User Accounts.
To change your own password, press CTRL ALT DELETE and then click Change Password

restart your computer repeatedly tapping the f8 key to enter safe mode while the screen is black once in safe mode where the only things working will be your keyboard and mouse allowing you to attempt to repair your computer

Reboot the computer As the computer is booting repeatadly press the F8 key as the computer is booting

If done properly the user should get to a screen similar to the below screen.

Select the option for Safe mode command prompt only

Windows Advanced Options Menu
Please select an option:

Safe Mode
Safe Mode with Networking
Safe Mode with Command Prompt

Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Debugging Mode

Start Windows Normally
Reboot
Return to OS Choices Menu

Use the up and down arrow keys to move the highlight to your choice.

select safe mode with command prompt

Then type at the C:\> prompt: type

DEBUG

and hit Enter. You'll see a dash (-) at the DEBUG prompt. Now type:

o 70 2e

which will show as -o 70 2e at the DEBUG prompt. Hit Enter. Then type:

o 71 ff

and hit Enter. Finally type:

Q

and hit Enter one last time to quit DEBUG back to the C:\> prompt.
Now you're ready to reboot your PC, and press the appropriate key or key
combo (look it up in your motherboard manual if you don't know it) to access
your BIOS Setup.

Youll notice the password is gone.

if this fails try typing

At command prompt type debug
you will get a - prompt where you can type the following (<enter> means press enter, not type enter*)

A <ENTER>
MOV AX,0 <ENTER>
MOV AX,CX <ENTER>
OUT 70,AL <ENTER>
MOV AX,0 <ENTER>
OUT 71,AL <ENTER>
INC CX <ENTER>
CMP CX,100 <ENTER>
JB 103 <ENTER>
INT 20 <ENTER>
<ENTER> just hit enter on this line
G <ENTER>
Q <ENTER>
Basic BIOS password - works 9.9 times out of ten

if this fails you will need to

Select safe mode with networking to download

you could download all of these or just one save them to a usb drive for later use if one fails to remove the password

http://www.tech-faq.com/reset-bios-password.html

http://majorgeeks.com/CmosPwd_d239.html

A CMOS BIOS password recovery tool

Editors note: This tool should only be used by experienced computer users. Please see the enclosed read me if you would like to run it from Windows. Failure to follow instructions (from Windows) will cause your anti-virus to flag this as a virus.
hope this helps

when turn on your pc or laptop press F8 then Safe mode go to control panel look for user account then delete password to fix your problem