Power supply Playing Dead:
Sometimes a power supply will get "stuck" in a mode, even though it is not overloaded. I don't know what causes this, maybe a glitch/power spike or something, but the power supply will not turn on. "used to work fine before".
Solution: turn off the power switch on the power supply or if it doesn't have one, unplug it from the wall. wait 10 seconds for the caps to drain. turn it back on. if it powers back on your problem is solved. Before doing this I have tried disconnecting all the connectors & reconnecting just the mb to no avail. the solution was the only thing that worked. PC was fine after that. 400W PSU.
Another solution: Tripp Lite 8-outlet ISOBAR surge protector. $57. takes out noise & spikes.