This is often a symptom of a bad motherboard or cpu, but it can be caused by other things. I had the same issue and it was caused by a scratch across my processor which was near impossible to see without looking hard, replacing the processor solved the issue.
Some things to try:
Take everything out and reinstall it, checking the motherboard and cpu particularly for damage. Make sure you plug in the cpu power (easy to forget) and try again. Make sure there aren't any cables touching the motherboard.
Plug the display cable into your motherboard, without the gpu installed and try again.
Remove the CMOS battery (round battery on the motherboard) and leave it for 10-15 minutes. Press the power button on the case repeatedly to remove any charge, then reinstall the battery and boot again.
Remove all usb devices (keyboard & mouse included) and try to boot
If none of the above work then try borrowing a cpu and motherboard and see if they work.