My initial guess might be that you just tripped an internal breaker of sorts, and it simply needs to be reset by flipping the switch on the back of the unit, and pulling the power cable for good measure, for a few minutes. I'd say give it 5 minutes to be sure any residual charge in a capacitor has dissipated before trying again.
If that doesn't work, odds are the PSU is blown and you'll need a new one. Hopefully that's all that happened, and a surge didn't get sent down any of the other cables, frying some of the other components. Unfortunately, the most effective way to figure out if this happened, is to replace the PSU and see what still works.
I have an old pc here which the floppy drive wasn't reading the disk (the light was on so it had power). Like an idiot i pulled out the 4 pin lead and plugged it back in whilst the pc was still turned on and it shorted out. Now i have no power. Have i blown the PSU or could it be something else?
Amd Athlon CPU
trinity K7 m/b
32 Mb RAM
Any advice would be appreciated, Thanks.