swap in a new one that's known good. They do go just like that.
Tried to turn on my PC today, and it wouldn't turn on. Not a thing. No lights, no nothing. I'm guessing this means the PSU has given up the ghost, but there were no tell tale signs (like black smoke
). Just looking for confirmation that this can happen just like that, it is a pretty old PSU, about 5 years now.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic