you will have to put your iPod into DFU Mode and Restore:
# Connect your iPod to your computer
# Turn your iPod off
# For 10 seconds, hold both the Home and Power buttons
# While still holding the home button down, release the power button only! Continue to hold the home button down for an additional 10 seconds.
# After the dialog box explaining that the iPod was found in recovery mode has been closed, restore your iPod
If iTunes does not recognize the iPod, then you should reinstall iTunes
This video should help you get into DFU Mode
http://www.youtube.com/watch?v=bITIiGswjFI