Move the camera view display back to the center of the screen. Something strange happened yesterday when I was working on a model. The camera just moved to the right in preview, and I have no idea what I did, nor what to search for. I'm not really sure what to do to get it back to the center..
In camera-view, press shift + middle mousebutton, then move your mouse and reposition your cameraview.

Alternatively your keyboard might have Pos1, works for you as well.