Nikon camera d80 screen problem?

So my sister was fooling around with the camera snapping as many pictures as she could. Then suddenly she came to me and told me that when you press the button to snap a picture the screen remains blank. From then on we havnt been able to figure out what's wrong. The battery is full. The memory card is in.i've deleted all pictures. And on the top of the camera the thing says 'Err'

Remove the lens, see if the mirror is down. Reseat the lens