First, try a reset - to do this, press and hold the Menu button and Information button (these buttons should be marked with two small green dots). Press and hold those buttons for a few seconds.
If that doesn't work, then there is a small switch underneath the dial that can become gummed up. You can try spraying some electrical contact cleaner under the dial, to see if that helps fix the problem.
If it doesn't work then the camera will need to be returned to Nikon to have the dial switch repaired.