Just to add an anecdote, I have a 50mm f1.4 G Nikkor that exhibited exactly the same failure mode. Turns out the motor was fine, but one of the rings in the barrel had slipped / become misaligned causing autofocus to fail and manual focus to slip. It became accessible after some light disassembly and I was able to reseat the ring correctly. It focuses fine now, but you've got to worry about other effects like decentering when the lens was presumably dropped in the first place.