I recently upgraded my motherboard,CPU and RAM and after a smooth reformat and windows install my computer randomly shut off and would not power back on. I replaced the power supply but it would show life for a split second then nothing. I discovered that if i don't plug the PCI-E connectors to my videocard the computer boots up with all fans going and I can hear my hard drive starting up but no signal to my monitor through my onboard video.
If the video card has be tried and tested in another computer, the problem is most likely a faulty motherboard.
If your mobo has several PCIe slots that plug in the video card, try those as well, could be that the main one is broken.
the on board video wont work unless the pcie card i actually removed. Pull the card out and see if the computer works correctly. Clean the card and blow out the PCIe slot with compressed and and put it back togehter.
If that does not work it could be the board (PCIe Port died) or more likely the video card. Do you have a spare video card or a friend that you can borrow one from for testing?