Could also be a hardware issue. I had a faulty A8N-VM CSM motherboard that had only 2 working USB ports. The other 2 at the back didn't work and neither did the front panel ones, out of an internal header.
First things first, try turning off your computer and unplugging it for at least 5 minutes. Then, take out the battery on the motherboard (it's a small flat one), then put it back in after a little while. Remember to ground yourself on either the case or the power supply housing while doing this.
This might fix it, report back if it doesn't.