Connect the iPod to your computer and restore via iTunes.
iTunes: Backing up, updating, and restoring iOS software
If problems see:
Try:
- iOS: Not responding or does not turn on
- Also try DFU mode after try recovery mode
How to put iPod touch / iPhone into DFU mode « Karthik's scribblings
- If not successful and you can't fully turn the iOS device fully off, let the battery fully drain. After charging for an least an hour try the above again.
- If still not successful that usually indicates a hardware problem and an appointment at the Genius Bar of an Apple store is in order.
Apple Retail Store - Genius Bar
and maybe
iOS: Device not recognized in iTunes for Windows
or
iOS: Device not recognized in iTunes for Mac OS X