Try holding down the play/pause and middle button at the same time for roughly 10 seconds. This should force the iPod into emergency connect mode in which case iTunes should recognise it and you can try restoring it.
If this doesn't solve the problem you'd be best served reporting the problem to Apple so they can help you out.
In addition to what Elliot said, if you happen to have an iPod touch,long press the home screen button and then the sleep button simultaneously.. do this until you see the apple logo again.. that means an emergency reboot.
and again , if that doesn't solve it, you probably have a defective unit and probably Apple can help you with that.