My daughter was playing with her 5th gen iPod Nano and somehow locked it. Now whenever the screen comes on there is a picture of a padlock. I have read that you can connect it to the "original" computer that the iPod was set up on, but it was so long ago I'm not even sure that computer is around any more. In any case, it says this is only necessary if you have forgotten the unlock code.
Update: After fiddling with it for too long, I discovered that if you hit the round circle in middle, it pops up a four digit code for you to enter. But I have never entered a code, my daughter claims she didn't either, I don't know what computer it was originally set up with, and to add insult to injury, I don't even know where the cable went that connected to this thing.