Gateway 507GR wont start..

Hello,
I've had my Gateway 507GR desktop for almost 5 years now, NO problems until now.. I was sitting on it the other night and all of a sudden the screen went blank and the fans seemed to stop: they're very loud always have been so it's obvious when they stop.. So I shut the computer down and waited for a few minutes and pressed the button to turn it back on, the blue light on the start button turns on, no fans start up, no noises from inside the computer at all, and the monitor doesn't show anything. Went and got a new power supply hooked it up, but it still does the same exact thing, except every once and awhile one fan will start and stop within a second of hitting the power button. I have no idea what else the problem could be, any ideas would be really helpful!