Try resetting it, hold the middle and top button down together until it restarts. If that doesn't work then I'm sorry, but you'll have to get a new one because the chances are that you won't be able to fix it without professional help. The new third generation iPod Nanos have been experiencing a lot of technical difficulties, especially if yours is among first batch that was made, and they've smoothed out most of the glitches so far, but they do turn up occasionally. Mine at first had a problem with the sound, you could only hear the harmonies and the music, but no vocals. I went to the Apple store and they said that they had a lot of iPods being returned because of faulty features and glitches. The said they couldn't fix mine, so they gave me an entire new one. If you're good with electronics then maybe you can try opening it up and seeing if there's something wrong with the soundcard. What I think happened is that the part that connects the headphones to the jack somehow shifted and only the left/right wire is in contact with the other. Headphones work like a circuit, a current carries the soundwaves to your earbuds and makes the music. If the wire isn't touching, then the circuit is disconnected and the current can't get through to one ear. The other one seems to be intact.