Found the solution on another site (http://www.everythingusb.com/forums/showthread.php?threadid=11238
). To quote user AFHavemann :
"* Plug in the player so the USB bus can see it.
* Once it’s recognized (i.e., the device screen reads "Charging" and "USB Disconnected"), go to the Control Panel and click "System
* Choose "Hardware"
then "Device Manager"
* Look for the name of your device under “Portable Devices”
in the list
* If the name of your player is under “MTP Device”
then the correct drivers are probably loaded
* Right-click on the name of your device and click "Properties
* Click “Update Driver”
Then follow the prompts from there. The computer searches for the driver software, etc., and reinstalls/resets information related to the device, which fixed things such that the computer will now recognize the device. I'm not sure what causes the problem, but this solution seems to reset whatever's gone wrong.