The gear that propels the shutter release may be sticky.
Or the motor that drives that gear may be getting weak.
Or the shutter itself may have a problem with releasing.
There are things you can try, but I'm sorry to say that this camera probably needs to go to Nikon Repair, which can be engaged (or located) through this page:
You could also try, before going to Nikon Repair:
- removing the batteries, and leaving the camera on a (dry, non-humid, non-dusty) shelf for overnight. Then put the batteries back in and try the camera again.
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