This happens to me more often than I like. Steps that generally solve the issue (after each step, try plugging it in):
1. Try different port. (try all of them)
2. Restart computer.
3. Restart iPod.
4. Step 1.
5. Re-install iTunes.
6. Steps 2, 3, 1 (in that order).
Those should work. Try them in the order given first.