Howard5252 wrote:
SOLUTION FOUND:
MartinFisher pointed out that the AF-ON button can be re-assigned and suggested that I make sure it hadn't been. It turns out that while assigning some other buttons, I turned off the AF-ON button (the None option). Sorry for all of the fuss, as is usually the case - it was an operator error. Thanks to all who posted, this site does have a wealth of knowledge and I'm glad I posted my problem before sending the body off to Nikon.
It’s amazing how often in one way or another we change a setting without intending to. Glad you got it worked out!