Shun Cheung , Sep 24, 2008; 11:17 a.m.
Did you accidentally switch to manual focus either on your camera or on your lens? Right below the lens release button in front, there is this switch to select auto focus and manual focus.
The image below is from a D300. The D80's switch has only two selections but is similar.
If that is not the problem, I would reset the camera to see whether the problem clears. After that, you can take the battery out overnight to see whether it clears.