Yeah unplug everything, hdd's, gfx cards, cd drives. Only leave mobo in there. When you hit the power button on the pc, anything happen? Lights flicker, fans spin momentarily? Only way you're gonna be able to test ya PSU is to use another one to see if the same results. Things like this happen all the time. Especially since the fucker never gets shut down like that, going cold could have caused it to just kick the bucket after being live for so long.