Why is the image in the viewfinder a circle in the centre not a frame?

My D5200 is weird! The image in the viewfinder is a circle in the centre of the full frame. The outer area is dark. This happens with different lenses which seem to be fitting properly. Oddly, this occurs when the battery is inserted, even when the camera is swirched off. Before inserting battery the image is correct, and the pictures are fine BUT as soon as the battery goes in I get the strange circular view in the viewfinder. Does not change when camera is switch on. How can this be possible and what do I do?