5+ years ago I built my own pc, which is fitted in an Antec 900 case. It has been working without issues ever since.
For the entirety of that period of time I've been using USB-based headphones, though I have used the jackstick on the front for audio output.
Today I plugged in my headset without the external soundcard attached, which means I plugged the jackstick for both audio out and mic in. Upon plugging them both in my PC shutdown immediately. All lights on the fans turned off and only the green light showing power was still connected showed.
I removed the headset and turned off the power, but even so my PC wouldn't start up. I have tried unplugging the cords for the front panel (both audio, power, reset and USB) and replacing them, and have kept the PC without power for several hours, but nothing seems to work.
The most peculiar thing is that if I plug in my headphones now the lights on the fans turn on and the fans move momentarily, before stopping. Like a jumpstart. The same seems to happen even while the cords are unplugged, although not when the audio cords are plugged in.
I've also checked the motherboard for blown capacitors or other obvious burns, but have not seen anything.
I'm a little at a loss for what else I can do, so I'm hoping someone else has a good idea for what else I could try, though I'm slowly dreading it's down to a replacement of either the case or the motherboard (more likely).
I have seen several threads on audio issues (static and grounding issues) as well as USB issues, but none seem as severe as this.
Sounds like the mic/headphone jack has come loose and a hot lead got grounded. Try removing the jack leads from the mobo header and try it.
I've opened the top panel and attempted to check the jacksticks, but they've been implemented in a plastic "brick" containing the USB and firewire as well. I unplugged all of these and the ground connection to be certain, but I'm still not able to boot. Sadly I think the motherboard has been damaged, and will need a replacing.. as will the chassis since it seems to be the one at fault..