unplug everything except memory, CPU, and video card. try again.
if there's still nothing, take the CPU out and try powering on again. if you hear beeps, it's probably a blown CPU. ditto for the memory. if nothing works, it could be the system board.
if there's still nothing, take the CPU out and try powering on again. if you hear beeps, it's probably a blown CPU. ditto for the memory. if nothing works, it could be the system board.