|Does it give any BIOS beep when you try to start it up?|
Remove all the ram and then try to boot up. If the CPU is ok you should at least get a bios beep code for ram error. That will at least tell you if the CPU is working.
If it passes that test re-insert 1 stick of ram and try again. Any beeps? Any display?
If you continue to get no display after working out whether or not the CPU is working it may be a dead graphics chip on the motherboard.
Goin' Fishin' (Some day)