Monitor screen get black while cpu still run

Registration date
Monday August 20, 2012
Last seen
August 20, 2012
while working on computer my monitor screen suddenly get black and my monitor works fine on another pc.
when the problem occures i can see
1. power LED light still on
2. HDD LED light get off
3. processor fan still runs
4. keyboard not works
5. Restart button not work

then forcefully i have to turn off the main power.
if i keep the computer off for 1 day, next day it works for 4 - 5 hrs and then the same problem occurs. again if i turn
on the computer just after few minutes then i can see only the
1. power LED is on
2. processor fan is running
and beep sound is not heared so monitor dont show display
2nd time it starts and everything goes fine for just 1-2 minutes and then monitor again get black.
i've visited to hardware engineer but he says no problem is in motherboard.
plz help me,
which hardware is damaged