Autofocus has stopped working

The lock that's around the 4-way button that moves the AF point will stop it from moving. The AF switch (says C,S,M) on the body below the lens release button gets knocked to manual and turns off the AF

Set the camera to "spot focus".

You will need the interupter replaced the iris is closed down so the a/f will not lock on

You might like to check if the A/M slider switch on the lens by the Nikon label is set to A.
It sometimes gets moved accidently.
Set to M the lens will not autofocus.

I figured out that when changing lenses my finger must have hit the little switch and flipped it over to manual focus. I changed it back and it now works. Thanks.

The first thing to check is that autofocus is enabled on the camera body.

The auto/manual focus selector switch is located on the front of the camera, just below the lens release button (on the left side as you look through the view finder). Make sure that the white dot on the selector lines up with the white dot under the AF (autofocus).

If this does not solve the problem, make sure that your lenses are set to autofocus mode as well.

Start off with the simple things, does it work with another lens, is the AF/M switch in the correct postion for autofocus (AF), are the batteries good.

If all of the above are in order it maybe either the DC/DC converter on the bottom of the camera, this supplies power to various parts of the circuit. or it could be the motor drive transistors, these are located under the front cover and are located where you finger tips rest on the right hand side as you hold the camera.

Both these items are not really user changeable and I know the DC/DC units have not been available new for many years now.

I suggest you have a look for a friendly camera repair shop to pin point the exact cause.

You will have to keep in mind the age of the camera 23 years old parts are a big issue

Look on the left side of the lens. Make sure the button is on A for auto focus and not M for manual. OR not stuck in the middle. Hopefully that will solve the problem.

Are you depressing the shutter button half way prior to snapping the shot you want. Also check to see if any of you setting are changed...if it's on auto and you still are have the same problem then your lens could be dirty.

It is likely that the lens will need parts. Sigma is not the best for repair parts. Typically, the small nylon gears that control the AF break. It is still a useful piece in manual focus. Repair costs would likely exceed its value.