Most lenses include an AF/MF toggle, and if you bump the switch without noticing, your lens's AF system will suddenly be unresponsive.
Some Nikon cameras feature a back dial that encircles the multi-selector; as you can see in the image below, this can toggle between a dot and an L: It's known as the Focus Selector Lock switch, and it lets you determine whether you can select AF points using the dial.
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