My pc beeped twice and nothing came

Lack of video signal is usually 1 of 3 things, Power supply (PSU), Video card or Motherboard (mobo).
Situation #1: If you get a blank screen and no sounds or movement it either indicates that the power supply unit (PSU) has failed or something on the motherboard has failed, preventing normal startup.
Situation #1A: You might get some power lights coming on and some fans spinning but the video output does not happen, usually indicates that motherboard or PSU is going out or has gone out. Occasionally it will be due to a weak (damaged) PSU.
Situation #2: If you get sounds (normal Windows startup sounds) and lights but nothing on the screen then it indicates a failed or loose video card.
Always try a different power outlet that you know is working. (Eliminate the easy stuff first!)
If it is out of warranty, try opening up the case, with the power off, of course. If you have Sit #2 and have a video card that you can reseat (take out and then put back in) try that. If it is onboard video then you may have to buy a video card for it.
Also take this time to inspect the capacitors on the motherboard (they look like tiny soda cans) and check for tops that are bulging, swelling or leaking anything. Capacitors should be perfectly flat on top. If you find these then your motherboard is bad (well technically the capacitors on your motherboard are bad). The bad caps will need replacing to fix (not a job for an amateur). If it passes the visual inspection then it may just be your power supply is bad. Notice that a bad PSU does not mean you are getting no power, just not enough to start and run the computer.
A few years ago a company stole the recipe for making capacitors and started making their own capacitors at really cheap prices. The stolen "recipe" was not complete and not correct so these caps started failing after about 2-3 years of use. Emachines, DELL and Gateway were the computer companies most affected by the bad capacitor issues, although any of the big companies could have the same problem. These bad caps also affected power supply makers.
Do you hear anything else during startup, beeps, fans or drives spinning, lights on the front or back of computer flashing or lit up? Do you hear the Windows starting sound?
Let us know more then we can troubleshoot better. Do you have the 4 diagnostic (A,B,C,D) on the rear of the machine? What is the front power light on the CPU doing?

Sound like it could be your Rams, have you got a bleep cold book when you bought the computer, or if not you should beable to down load Gide from the Internet

Listed below is full explanition of Beep Codes for Dell Dimension 2400 system. I hope this will help you with resolving this error:

Beep Codes Your computer might emit a series of beeps during start-up if the monitor cannot display errors or problems. This series of beeps, called a beep code, identifies a problem. One possible beep code (code 1-3-1) consists of one beep, a burst of three beeps, and then one beep. This beep code tells you that the computer encountered a memory problem.
Reseating the memory modules may fix the beep code errors in the following table. If the problem persists, see "Contacting Dell" in your Owner's Manual for instructions on obtaining technical assistance.
Code
Cause
1-3-1 through 2-4-4
Memory not being properly identified or used
4-3-1
Memory failure above address 0FFFFh

If you hear one of the following beep codes, see "Contacting Dell" in your Owner's Manual for instructions on obtaining technical assistance.
Code
Cause
1-1-2
Microprocessor register failure
1-1-3
NVRAM
1-1-4
ROM BIOS checksum failure
1-2-1
Programmable interval timer
1-2-2
DMA initialization failure
1-2-3
DMA page register read/write failure
3-1-1
Slave DMA register failure
3-1-2
Master DMA register failure
3-1-3
Master interrupt mask register failure
3-1-4
Slave interrupt mask register failure
3-2-2
Interrupt vector loading failure
3-2-4
Keyboard Controller Test failure
3-3-1
NVRAM power loss
3-3-2
NVRAM configuration
3-3-4
Video Memory Test failure
3-4-1
Screen initialization failure
3-4-2
Screen retrace failure
3-4-3
Search for video ROM failure
4-2-1
No time tick
4-2-2
Shutdown failure
4-2-3
Gate A20 failure
4-2-4
Unexpected interrupt in protected mode
4-3-3
Timer-chip counter 2 failure
4-3-4
Time-of-day clock stopped
4-4-1
Serial or parallel port test failure
4-4-4
Cache test failure

1. is the keyboard connected correctly?
2. did you change ram or hardware recently? a pc often halts when u have different rams on each slot, had a vaio with a slot with 256 and another with 512 and ALWAYS did that but on any key press it started

mostly could be keyboard...