did you change the ide leads around or change any jumpers on the hard drives?
Try disconnecting all the hard drives, and cd players, and see if you see a boot up screen.
When you switch on, how many BEEPS do you hear?
It could be you accidenatally dislodged something. My pc has a video card in the agp slot, and its very temperamental, one tap and I can lose the video signal. However, this gives one long beep followed by 3 short beeps.