My desktop went tits up today and I'm trying to figure out what went bad.

I was watching a flash video when the system turned itself off and rebooted. Ever since it hasn't worked right. The computer will turn on (fans come on, the system beeps, the HD drive light flickers a few times), but it won't boot and my LCD screen keeps saying "No Signal". I tried swapping out the graphics card, but that didn't seem to make any difference.

Could it be the hard drive?