Check to see if all connections to the motherboard are there. It sounds like you are not giving the CPU / Mobo any juice...
A friend has a Gateway 834 tower system with XP. When powered, it sends no signal for the monitor. The monitor works fine on another computer so I know that it is not a monitor or cable issue. The owner told me that she had taken it to a small computer shop (that is closed now) where they "re-energized the motherboard". I don't have a clue what they might have done.
What could I do to get the display back? I can't reload the drivers since I can't see the screen? Does anyone have any suggestions?