You must a follow the next steps :
1. Turn off your iPod.
2. Hold on the menu button for a 5 secs.
3. While your are pressing the th button connect the iPod to the USB cable already connected to the PC with the iTunes opened.
4. iTunes send you a message that says: "iTunes has detected and iPod in recovery mode an you have to restore before using with iTunes".
5. Just wait a little to get the iPod restored.