I've been trying to sync my iPod for the past hour and it won't! It shows up on itunes, but I get the "this ipod is corrupted" message. Then it asks to download new software in order to restore, but I know I have the latest cause I checked. Also, it will not turn off. I'm wondering if its useless now.
Try holding the center button and the "up" part of the clickwheel down for a few seconds. This will reset it (your songs will still be there).
If that doesn't work, try restoring the software via iTunes. You will wipe out your music, but you can always reload it from your hard drive.
If it's still under warranty, Call AppleCare.
A last option that works (but is controversial) is to drop or smack your iPod against something. Mine (4th gen. clickwheel) started coming up with the "sad iPod" icon last Thursday. I read about this fix online and tried it. I dropped my iPod onto a cardboard box from about three feet up. It actually worked! My iPod works fine now.