You will need to restore it in recovery mode. To do this, unplug your iPod if it is plugged into the computer. Then, power off your iPod. Now, plug it in and as soon as you plug it in to your computer, press and hold the home button. A screen with the iTunes logo and a cable will appear. iTunes will say that there is an iPod in recovery mode and it needs to be restored.
Note: A restore will erase ALL data on the iPod.