I have the same problems sometimes. You need to go to the I-Pod updater which came with the I-Pod software. You need to then reset your I-Pod through the software. Then un-plug it from your computer, then plug it into an outlet to charge for a few minutes. Then... restart your computer, and it SHOULD work, but if it doesn't, re-set your I-pod a different way by holding down the Menu and The Center button and hold them untill the screen goes blank, and the "apple" appears.
If it doesn't work, either repeat, re-restart your computer, or take it to bestbuy or another computer place. But... when you call apple for their help, they charge you like 40 bucks for a 5 minute convorsation.