1. Press "Info" button and check the AF setting. It should be "AF-A" or "AF-S" and not "MF". If not, use "i" button to set it back to "AF-A".
2. Check the switch on the lens. It should be at "A' and not "M".
3. Switch off your camera, unmount the lens, mount it, and try again.
If all of the above doesn't correct the problem, you'd probably need to make a trip to NSC.