Over the past several months on a number of occasions my D800 will not AF - it won't even try. No sound from lens or camera and no focus hunting. This has happened with my 20-70, 70-200 and 80-400 on several occasions for each lens, so I am confident the problem is not my lenses. However, it is not a common occurrence, which is why I've waiting several months to post - I wanted to be certain it was real. The only way I can get the system AF-ing again is to remove the lens and remount it.
I will admit that on several occasions I "may" have changed lenses without turning off the camera first, but on my recent trip to Africa, it occurred without any such provocation, and I'm sure I had done the same with my prior D300s and NEVER had the camera fail to AF.
Switching from M/A to M and back, pulling the battery, manual focusing to move the elements, checking the focus lock button, etc. do not resolve the issue. The lens has to be removed and remounted.