Do you have any other devices connected via USB to your PC? Are you plugging the iPod into a high powered USB port located directly on the PC versus into a hub, etc.? Maybe try a different USB port to see if that helps. It wouldn't hurt to also try a different USB cable as well to rule out a faulty cable.
If still no luck, try forcing the iPod into Disk Mode using the instructions below and then restoring the iPod as it may be an issue with the iPod's software.
Putting iPod devices with a Click Wheel into Disk Mode
- If necessary, verify that your iPod has a Click Wheel.
- Before placing the iPod into Disk Mode, verify that it is charged.
- Toggle the Hold switch on and off (set it to Hold, then turn it off again.)
- Press and hold the Menu and Select buttons for at least six seconds, until the Apple logo appears.
- When the Apple logo appears, release the Menu and Select buttons and immediately press and hold the Select and the Play/Pause buttons until the Disk Mode screen appears.
- Connect the iPod to your computer; the screen on the iPod will should say "Do not disconnect".
If you are having difficulty putting your iPod into Disk Mode, set it on a flat surface. Make sure that the finger pressing the Select button is not touching the Click Wheel. Also make sure that you are pressing the Play/Pause button toward the outside of the Click Wheel and not near the center. If you are still unable to put your iPod into Disk Mode, use one finger from one hand to press the Select button, and one finger from the other hand to press the Play/Pause button.
Disk Mode should help the iPod stay mounted as a drive on your system long enough for you to perform a restore of the device in iTunes.