NooooooooooooOOOOOOOOO!
Gutted matey.
Have you tried changing the PSU or testing it with a multimeter? Chances are that swapping the cables round has caused a short between 12V and something (hopefully only ground). With luck this may have just 8uggered the psu, but it could have burnt traces, components, or even ICs.
Also, I'd examine the power header on the motherboard - these are (apparently) notorious for being not up to taking high current. Even a loaded system can cause them to fail over time, so a 12V-ground short most definitely could. Look for any discolouration or oxidation on the metal pins or plastic shroud.
If that doesn't work I'd recommend that you disassemble the motherboard from the tower, and examine both sides to see if you can find any burnt components/traces.
Here's hoping it's only the psu :-(
