Look closely, the pin is probably pushed back or bent and shorting on the metal casing for the usb port. This happened to me with the front usb port (kids tripped over the Rockband Drum cord)and all I did was straighten the pin with a small pair of tweezers so it wasn't shorting and voila, it started again. Now I don't use that port cause the plastic guide that the pin was attached to is missing, plugging anything in again would just bend the pin again.
Hope this helps
Gamertag
Rzee350