My DSLR's autofocus doesn't work after I dropped it — how can I narrow down the problem?

I know you don't want to hear this, but the odds are super, super strong that the problem is something physical caused when you dropped the camera. Even with padding, there's still force, and it's possible for the camera to land badly.

The factory reset you performed rules out most common possible configuration problems, and you've gone through other sensible diagnostic steps.

So, the next thing to do is send the camera and lens in for repair. This will probably cost a couple of hundred dollars — but, hey, still cheaper than a new D7100.

I suggest sending both camera and lens, because it's very possible both were damaged — maybe in ways other than the autofocus. I guess the exception would be if this is the cheap kit lens and you were planning to upgrade anyway. In that case — I guess this is now an even better time to pull the trigger on that.

If the camera is communicating with the lens (i.e. if the camera isn't showing an error and refusing to take pictures and/or shooting massively blown out images at narrower apertures), then there are really only two possible parts that can realistically be bad:

• The focusing motor/gears in the lens.
• The focusing sensor in the camera.

If your camera has a live view mode, you can use this to determine definitively which one is at fault, because live view mode does not use the focusing sensor.

Otherwise, your only option is to try a different lens, and statistically, that's probably what is broken, so that's not a bad idea in general. In particular, if your lens is not internal-focusing (i.e. if the end of the lens moves when in autofocus mode), you almost certainly broke the focusing motor on impact. Given that most higher-quality lenses are internal-focusing (for precisely this reason), this is, as other folks have suggested already, a good opportunity to upgrade your lens.

Note: If you have a really old Nikon lens that requires the in-body focusing motor, the problem could also be the in-body focusing motor. In that case, the live view test could be somewhat inconclusive. Either way, consider upgrading to a newer lens. :-)