1 USB port not working...

http://support.apple.com/kb/HT1151

OS X Mountain Lion: Reset your computer’s PRAM

If above does not work, take the computer to an Apple store or an AASP. Whichever is more convenient for you. It's possible you have a hardware issue.

I tried resetting the PRAM, didn't work, but repairing disk permissions after that fixed it for me too. Thanks!

Similar problem - MacBook Pro 15" Mid 2010 had one of two USB ports stop working. No other faults in performance. Apple store evaluation found traces of brown gooey substance leading from fan to the non-working USB port with burned leads (shorted out). No spilled beverage events known. Out of warranty repair would cost $1240 (motherboard replacement). The only explanation I can think of is an insect crawled into the fan when not operating and was killed when fan ran - leaked down along case to USB connections to motherboard and shorted across leads. Going to have to try home repair effort - any experts have an opinion on my chances of success?

Examined motherboard and there was a trace of "brown goo" leading from fan to malfunctioning USB port connections - sprayed with contact cleaner and brushed gently. Let dry and closed up - no change. No losses. Will continue with one USB port remaining. When / if I migrate to a new MacBook Pro eventually, I expect them to have done away with the multiple ports in favor of one super port; the lack of redundancy concerns me considering this experience with a mysterious brown goo appearing inside the machine. Perhaps the lack of a fan in the new designs will mitigate the lack of redundancy, the presumptive explanation being a bug crawled inside via the fan while dormant and was pulverized when it started turning, leaking onto the motherboard. No other good explanation.