If your computer was powering on but you had nothing on the screen, my guess is that your video card had died, connecting it to the intergrated (onboard) video probably did not work because it was disabled in the bios due to having a seperate video card installed, uninstalling and reinstalling the memory and processor would have no effect on the video and it is possible that while doing so you have created a seperate problem.
If your computer does nothing at all,
The best way to troubleshoot this problem is start at the beginning of your power source, check to make sure there is power at your wall outlet. If no, check for blown fuse or breaker tripped. Also check that correct voltage is selected on back of power supply and that the on/off switch on back is set to I and not O.
Check that the fans on the power supply come on when you press the power button? If not, probably bad power supply, could be bad power button.
If fans are on, check for any lights that light up on the motherboard, check for case, processor and video card fans running if yes to all, still could be bad video card.
If possible take video card out and try it in a different computer, if it works in another computer and the video card requires a seperate power connector it could still be power supply has bad rail, maybe try using a different connector if you have a spare available and it's on a different rail. Could also be just needed to be reseated and that is why it worked in another system.
hope some of this helps...
