Re: Big problem.. with computer
Erm, start with your PSU. You could have blown a fuse (if they have one in, I can't remember). If that doesn't work at all then that may be your problem. As for frying everything... did you turn the voltage up or down? If it was down, I don't see how anything could be wrong but if it was up you may have poured way too much power into it and thus could have fried one or all of your boards and cards.
But yeah, put the voltage back where it was and try a new PSU first.