my 2nd generation shuffle not appearing in itunes. What can I do?

If you have not done so already, first try using a different USB port that is directly on the computer (not a hub).

If that does not help, I would shut down (completely power off) your computer. Disconnect the shuffle and do this reset procedure

http://support.apple.com/kb/HT1655

Start up your computer. Run iTunes. Try connecting the shuffle again.