What could cause a D7000’s mirror to flap and "Err" to be displayed when trying to take a picture?

When I press the shutter on my Nikon D7000, the mirror noisily flaps up and down — nothing else happens except
Err in display. It’s not possible to take a picture and the
Aperture and Shutter wheels are not operating.

What caused this? I sold the camera and it was brought back next day in this sad state!