Fix your issue with these steps:
1. Try forcing the lens to rotate a fraction of a inch. This will sometimes help the problem. Or simply move the switch from M to A on the lens.
2. Try to switching to a single of the 11 auto focus points. Then try if there are specific points that are the problem.
3. If all auto focus points are fine, try updating the firmware or cleaning the sensors inside the camera behind the mirror. If a one or more points are out of order, also try cleaning the sensors behind the mirror.
4. Detatch the lens from the camera, by applying pressure on the release button and twisting the lens so it can be released. See picture
5. Set the camera selection wheel to manual (M), see picture
6. Turn the camera on, and start the liveView format. This will flip op the mirror inside the camera.
7. Take a dry can blower. Or if in lack, use an empty plastic bottle, without moisture inside, and use it to blow on the autofocus sensors.
They are located at the bottom under the mirror. NOTE Do not blow with your mouth, small amount of moisture will hit the CCD also located here.
8. Turn the camera off
9. Reattach the lens. Push the lens, white point to white point and turn the lens in place.
10. This worked for me, hope it also helped you ;)
For more solutions check Nikon D80 Manual