computer does not display anything

Try the monitor on a different computer. If it works there, then the computer has a problem...

What OS is on the computer? Do you have a boot disk? Do you know whther the hdd is working? Can you boot into Safe Mode (Press F8 and see if you get anything to appear)? Open the computer (powered off) and check/verify that all the cards in the computer are properly seated and connected. Make sure that the cables to the hdd and floppy drive and cd-rom, etc are all secure and have not come loose. Reseat the RAM. Try booting again.

If all you are getting is the power lights from both the video and the computer, then the power supplies are probably okay. I'd suspect a hdd problem or a systemboard problem if you cannot get beyond thepower lights.

What are the specifications on this system?

Insuficient Data in question. High possibilities are: 1 bad video card. 2 bad hard drive. 3 no beeps on boot is bad MB or CPU or RAM.
If you are not experienced PLEASE take to local PC shop.
If older PC buying new one probably cheeper.Take to qualified Tech to Clone old drive to new drive.

Make sure that the video card is seated properly on the board. If not, nothing will show up on the monitor and 9 times out of 10 you won't get a post code.

How many beeps do you hear during boot up? 1= normal operation, 3=video card problems. Also unhook video cable from monitor to pc, you should see self test menu to rule out pc problem. It should tell you that you have no signal if monitor is good. Have you attempted to degausse monitor from it's menus? Got another monitor or one that you can switch out or borrow from a friend/family/neighbor? Lastly follow the other's advice, open pc reseat video card due to possible chip creep, reseat ide cable to hdd, etc... Did you have this pc before or did you put it together recently or replace any parts? Provide more info for better assistance, luck.

I have WIN XP Prof
I already tried F11 or F8 nothing displays.
also tried with boot disk.