Is your camera actually focusing when you depress the shutter release button halfway?

Look at the lens, does it have an M - M/A switch? Check to make sure it's not switched to M. This would mean the lens is in manual focus only.

Also on your camera there is a switch on the left side of the body that says C S M - if it's on M, this is also a manual override for AF lenses but also disables focus on AF-S lenses as well.

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