I pulled out a usb cable and now the motherboard's fried

I just pulled out the USB cable that links my iRiver mp3 player to the computer, and my computer shuts off and won't boot up. The monitor doesn't even turn on--the light in the front of the computer just stays on amber and the power supply's fan runs. This was with one of the two front USB ports. I put in my old motherboard and it now boots up, so it's definitely motherboard related. What the *12#!@ could have happened? I didn't do the "safely remove hardware" thing but I rarely do and it has never given me trouble before... if this is it, then I'm definitely doing it in the future.