My Macbook pro turns on but then gets stuck at the Apple logo with the loading gear and never finishes loading.

SOURCE: APPLE MACBOOK

If you get the Apple logo with spinning lines underneath it but not getting past this stage then the hard disc is being seen but something is failing in the boot process, causing it to stall.
Most likely this is a case of incorrect file permissions which is easily solved.
There are 2 ways of doing it, depending on whether you have an OS X install disc to hand or not.
IF YOU DO HAVE AN OS X INSTALL DISC:
Insert the disc into the DVD drive, power on the Mac on after hearing the chime hold down the C key. This forces the Mac to boot from the DVD. After a few minutes you'll get a screen asking which language you wish to use as the main language; select English or whichever is appropriate for you.
On the next screen the Installer box will appear. Ignore this and instead on the menu at the top of the screen select Utilities -> Disc Utility. Once this has loaded select your hard drive on the left hand pane and then first click "Repair Disc" and once that has finished click "Repair Permissions".
Once these have completed successfully reboot and OS X should load up fine.
IF YOU DON'T HAVE AN OS X INSTALL DISC:
This is a bit more tricky. You have to boot into single user mode. To do this power on the machine, and after the chime hold down Cmd (Apple key) and S together. The screen will turn black and an a load of white text will start appearing and scrolling upward as the system loads. Basically this is what happens behind the Apple startup logo, only in single user mode rather than multiple user mode.
After a few moments the text should stop scrolling and you should see a prompt which looks similar to:
localhost: / root#:
Note that localhost is the name of your computer so will possibly be different to "localhost". At this point type the following EXACTLY including spaces, capitalisation and dashes etc:
/sbin/fsck -fy
now press enter, then type:
/sbin/mount -uw /
now press enter then type:
diskutil repairPermissions /
and press enter. It may take a few minutes to repair the disc's permissions, but once complete you should have the 
localhost: / root#:

prompt again. Now type:
reboot
and hit enter. The system will then reboot and should boot into OS X without issue.

SOURCE: macbook start-up

most cases you only need to re-install the operating system.
try unscrewing the ram door in the battery compartment(will not void apple warranty) pull the tab to remove the hard drive then re-insert it,then piece back together.and power up to see if it is the same prob upon booting(that rules out a bad drive)if prob stays follow the steps to re-install the operating system

SOURCE: My MacBook will not load

Since you do not have the original OS CD's you will need to obtain a replacement copy of the OS from Apple, there are some corrupted files within the system and it will need to be reloaded.

Reloading the OS should not affect any of your data files, however be sure to read the screen prompts so that you do accidentally tell the OS to reformat your HDD. Once you have restored your OS files and get the system running again, you WILL need to reinstall your application software, so if you had say MS Office for Mac installed, you will have to reinstall it for it to work again.

Unfortunately, this is your ONLY solution if you want to recover your personal data from the system.

Hope that this helps you out. If you can, see if there is a Mac Users Group in the city you live in and try to borrow a copy of the OS from someone to reinstall it.

Cheers,

Keith

SOURCE: Grey Screen Apple Start up issue

It appears there may be a possible startup item or issue with your firmware, which may be causing this issue, so I am going to provide some procedures in order to help you resolve this gray screen issue. But first, let me help you remove that disc first:

font.f1 {font: 13.0px Myriad Pro} font.f2 {font: 14.0px Myriad Pro} Procedure #1 - Ejecting CD from drive
Here are some steps I recommend you try to resolve the issue with the disc not ejecting properly

• Reboot your Mac
• Hold down your Mouse button(s) prior to the Apple Chime
• Keep holding the mouse button down while your Mac boots up
• The disc should eject before you reach your Desktop
• When it does, simply release your mouse buttons.
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If this does not help resolve ejecting the disc in your Macbook's drive, you are going to have to take more of a command-line approach:

font.f1 {font: 12.0px Helvetica} -EJECTING DISC USING TERMINAL-

From your Desktop:
•Click anywhere on a blank space on your desktop. Now you should see Finder listed near the Apple menu icon.

• From the Finder menu, click on Go, then Utilities from the
drop-down menu. This will take you to your Utilities folder.

• Locate and open the Terminal utility
• At prompt, type: drutil eject

• Now press the Return key on your keyboard

(NOTE: If you have a CD/DVD drive with a tray, type: drutil tray open press the Return key on your keyboard )en
• This command line should eject the disc from the drive, unless there are mechanical issues with the drive within your Macbook.
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At this point the disc should be ejected from your Macbook. If this is the case, then I will now need to direct you back to Safe Boot mode, but with a little twist, from what you may have tried when you last when to Safe Mode:
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I am going to have you take your Mac into Safe Mode to see if we can't remove a possible folder(s) that may be affecting your Mac's start up function.

Try these steps:
-SAFE BOOT MODE-
• Reboot your Mac
• Upon restarting your Mac, press the Shift key.
• Hold the Shift key until you see a White Screen with Gray Apple icon and eventually a spinning gear.
• When you see the Spinning gear, release the Shift key.
• Allow the Mac to continue booting up. Also this may take a little while.
(NOTE: this may take anywhere from 5 to 15 minutes to load to the Safe mode screen depending on the performance of your Mac, but if it exceeds this time, shutdown your Mac and try just once more. It may have just frozen up the first time around for you.)
• You are going to enter the Safe Mode (which looks like your main desktop environment the only difference is certain start up items are disabled.)

• Once within the Desktop in Safe Mode
• From the Finder Menu, click Go> Go to Folder
• Type in this line: /System/Library/StartupItems
• This path, is going to take you to the System Library folder which has all of your Mac OS X Startupitems.
• Remove all of those folders to the Trash.

*Reboot your Mac normally and see if you can access your normal User desktop environment.

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Post back to me, if these particular Safeboot procedure does not help. Thanks!

Regards,

-Mactechtrainer

SOURCE: help with my macbook air, it shows only the apple

Power the machine down. Press the power button to turn it on. After you hear the startup chime, press and hold the shift key. This will trigger an extended file system check and a safe boot. Continue holding the shift key down until you see the spinner, then you can release it. If it doesn't boot completely after this, you will have to bring it in to an Apple-certified tech for repair.