Computer Help.

Yes Signman,
It could well be the PSU.
Even if the motherboard failed there would still be 12 volts to the disc drives, you would hear them running and the trays would open on the optical drives.
A working PSU is still active even when the PC powers down, the power push button switch on the front of a PC is simply a trigger for the PSU to run up to full activity.
If the motherboard power connector had come adrift then this may shut down the PSU, as some of them monitor a pair of wires in the plug, and unless they are shorted together by the motherboard then the PSU won't fire up at all. This is also why some PSUs won't power up away from a PC. Some also require a minimum load before they will operate.

If there was a short circuit on the output side of the PSU you would probably hear it pulsing on and off as it's protection circuitry would keep shutting it down.

I assume the socket outlet on the wall has proved to be working and you have checked the fuse in the plugtop and tried another mains lead, just in case.

I don't know how many variations are currently in use but you should take the PSU with you when getting a new one, just to be sure you get the correct MB connectors and enough drive connectors.

I hope this helps a little.
VM