Computer turns on BUT there is NO display - please help!?

I would say that it might be a bad video card. I would check if you have a video card that may be overheating. What I would first do is try to replace the video card. That is my only recommendation. Try to turn off your computer and let it sit for a few hours and then turn it on and see if it is continuing to not turn on. If it does not turn then the video card is broken but if the display comes on and then shuts off then you have a cooling problem.