I've had a D5100 for 8 years now. After having recorded some video last week, earlier today I picked it up for some more recording and the autofocus was simply not working (I noticed it with the 28mm f1.8). I checked every possible setting and everything was just as I had always used it, including the switch on the lens.
I switched the camera on and off, tried removing the battery and changing lenses. The only thing that seemed to work was attaching the 50mm f1.8 AND assigning autofocus to the AE-L/AF-L button on the back. The shutter release "half-press" focusing didn't work with that lens either.
Switching back to the 28mm, not even the back button focus worked, although when I pressed it I noticed a very quiet squeaking sound coming from the lens, as if the autofocus engine was struggling to function. A similar but less problematic issue happens with the 18-55 kit lens, which, since a few years ago, always seems to need a little "push" from my hand on the focus ring so that it can autofocus.
I'm well aware that the D5100 is an entry-level DSLR and that I've probably already abused its worklife with over 15k clicks and a lot of video recording, but I was really hoping there would be some kind of home fix that could avoid me having to take it to service and postpone its unavoidable death for even a few months, as I'm in the middle of recording an ad video.
Some help would be much appreciated!