Put the iPod in recovery mode and restore it. Just be aware this will delete all of your data in the process.
Here's how to do it. First, plug it into a computer. After this is done press the power and home button together. This will reset the iPod. After this is done and it's at the apple logo, let go of the power button but keep holding the home button. After this, it should be in recovery mode and you can restore it.
This answer was originally to another question.