Next time it happens, pay attention to the green (black?) dot on the lower left corner of the viewfinder. That dot tells you whether or not the AF target thinks it is locked on target. If it's not lit, you probably can't release the shutter. It's only broken if the dot is lit and you still can't release the shutter.
Chances are quite strong that there's nothing at all wrong with it.
_____ Brian... a bicoastal Nikonian and Team Member
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