Try this, connect the iPod to the main computer you use it with (if you use it with more than one, this is the first one it was connected to) and open iTunes. Then disconnect iPod from the computer, it should be unlocked:
iPod: How to use the Screen Lock
If you can't get it unlocked you'll need to restore it. Just be aware that restoring will erase the hard drive, reload the software and put it back to default settings, so if you have songs on your iPod that aren't on iTunes you will lose them if you have no back up. Once the restore is complete follow the on screen instructions and it should connect to iTunes and give you a prompt to automatically update your library onto the fresh installation:
Restoring iPod to Factory Settings with iTunes 7