Every now and then USB ports seem to go into never, never land. An interesting trick I learned is to shut down the machine and remove all power from the machine. You must unplug from the wall or any battery backup units. Wait at least 2 minutes, plug everything back in and try again. This has worked on a number of client computers whose usb devices just disappeared. Removing all power for a period causes the USB ports to lose any memory of what was plugged into them at the time. The drivers are still there, so a simple restart of the computer after removing all power has done the trick for me multiple times. A restart alone will not do the job.