"It's time to start swapping components. I'd start with the power supply first. No need to take the old on all the way out of the case, just take a known-working PSU and plug it into the motherboard. Start with the power supply, then try the video card, then the motherboard.
Once you find the faulty component, replace it and through your known-working spare back in a safe place."
As Russ suggested you should start swapping components, especially the power supply and the CPU. This sounds like either the motherboard or the CPU is gone.
