I just got a D780. I went out to my lanai and thought I would fire off a few shots. When I press the shutter release button, the mirror comes up and just stays there. It will not return to its starting position until I press the shutter release again. I don't believe that's how it's supposed to go. What have I done? BTW I tried manual at first, then A and S - same thing every time. Thanks for the help!