theres a few things that might be, one is the power supply to the USB might have failed or more likely, the driver has become corrupt.
Have you tried restarting the machine?
try re-installing the drivers for the USB.
One of the first things to try when something stops working is "System Restore".
Run System Restore and choose a date when you know the USB's were working. Of course if the power to the USB's has gone this won't help. It it's a software problem then it may.