My Nikon D3100 won't autofocus except on guide mode?

It used to focus on all setting (except for manual). The lens is switched to auto and I can't figure out why it won't autofocus.

All I can tell you is that it should auto-focus unless you have specifically set the lens into the manual focus mode

If you are not finding your answer in the user manual, you can call Nikon at 800. NIKON-UX

It's a settings problem I think. Perhaps you may want to read its manual guide again.

Switch the lens switch several times to make sure you are getting it in the Auto position. Then try the various focus modes as explained on page 55 of your manual. If you still can't get it to autofocus, call Nikon to start the repair process.

Reset it or use Full AUTO mode to try.

Be sure the focus mode is set to one of the AF modes, preferably AF-A. See page 55 in your manual.