Friend of mine has a rig that has just mysteriously "stopped working". I wasn't there when this occured, so I've decided to take a look at it.
Upon trying to turn it on, the fan on the back of the PSU spins for a quick moment and then stops. At this point I decided further investigation was needed.
Popped off the side panel. Notice a light on the MoBo is on, press the button to turn the machine on - the fan on the CPU spins for a quick moment and then stops as well.
I'm fairly sure it's the PSU but don't have a back-up to test with.