I have a Nikon D200 and the auto focus isn't

There's a small contact area that has to make the electrical contact between the lens and body in order for the autofocus to work. Check this on the body for any damage or corrosion that may keep the connection from happening. There is also a very small mechanical connection between the two pieces - the D200 has the autofocus motor in the body itself - check for a small shaft that may be blocked or damaged as well.