Did you smell anything funny like Ozone? It may simply just be your power supply gone bad. If there are no lights coming on on the motherboard, no fans or anything reacting then chances are its one of 2 things... Bad Motherboard, or Bad Power supply.
Easiest way to check is to replace the power supply... in most cases factory made desktops don't come with one that is worth a hoot ~ 250 watt so go out and get yourself a nice 300-500 watt power supply and stick that baby inside the case. If she turns on, you're good, if not then you know what comes next ... its fixable
You can also test the current power supply by purchasing a testing unit. Basically its a little brick with LEDs that you plug the power cords into and see if anything comes out. I think I saw one at a local shop for around $10. I'm tempted to buy one just to have on hand, you never know when you may need one of those suckers.
Edited by Crazy49er, 24 November 2010 - 05:08 PM.