Do a reset. I'm not sure which nano you have, but typically a reset involves sliding the hold switch to hold then back again then pressing the center and menu button simultaneously for 6 to 10 seconds. You may need to repeat this a time or two. Also make sure to be touching only the center and menu buttons, not any other part of the control wheel and push the menu button from only the outer portion of the control wheel, not toward the center. Check out this link to the apple support page where you can select your nano, though the process should be the same:
[http://www.apple.com/support/ipod/five_rs/|ZELDA]