My nikon d80 will not always work in auto focus mode, why?

My nikon d80 will not always work in auto focus mode, why?

The contacts between the lens and camera may have become soiled with fingerprints or something else. Try to clean the contacts. Cleaning instructions should be in the owner's manual.

Also available light can effect focus and thereby whether or not the camera will actually shoot.
If there's not enough light - a poorly lit room or low light situation, auto focus can't find enough
contrast to grab focus.

So clean the contacts yes, and make sure you have enough available light.

In autofocus mode, the camera must have focus lock (little green dot indicator in the viewfinder) before it will take a picture.

Your camera needs to see contrast in the focus area to obtain focus lock. If you shoot a solid color, like a blue sky or a smooth blank wall, your camera can't focus on anything and will refuse to let you take a shot. If it's too dark and you don't have the focus assist light enabled, you won't get focus lock.

Another thing that can happen is the lens electrical contacts may be dirty. If you're using the 180-55 or 18-105 kit lenses, they seem to have dirty contact problems. Rotating the lens gently back and forth, like you're trying to remove it, is enough to clean the lens contacts.