Try holding both the power and center button at the same time. This usually overrides any problems and shuts off the Ipod. If that works and it shuts off, power it on again like normal, and see if that worked. If it didn't consider taking it to Apple. If you bring it into an apple store they can usually fix/replace it for free or very cheap. If there's not one near you, send it in through mail.

try pressin the home button and the power button at the same time, and holdin the both down for a few seconds.