I had the same problem with my ipod nano 3gen. It would not sync with itunes or be recognized by my pc.
Here is what worked for me: while the ipod is plugged in and itunes open I had to reset the ipod by pressing the menu and center button at the same time and then when the apple showed on the screen press the center button and Play/Pause. This brings up "disk mode" your computer and itunes should recognize your ipod after a few moments. You can then press the restore button which will clear the nano's drive and it will automatically replace the deleted items back onto your ipod. Hope this helps for anyone looking for a solution to a frustrating problem.