Just to elaborate on roaminggnome's answer, heres exactly what you should do:
- Connect your iPod to the computer that you used last to sync it.
- When the iPod appears in the iTunes sidebar, click it.
- Then click the "Restore" button.
- When prompted, choose "Restore From Last Sync".
- The iPod should take a while to restore.
- When you turn your iPod on, it should not be diabled anymore.
*Note: If this does not work, revisit Step 4. Then, instead of choosing "Restore From Last Sync", choose "Set Up As New iPod". BUT CONSIDER YOURSELF WARNED: THIS WILL DELETE ALL OF YOUR DATA INCLUDING MUSIC, VIDEOS, APPS, AND MORE!