Black box in middle of screen

So while watching tv sometimes for hours or minutes every once in a while a black box appears in the middle of my screen leaving about 2 inches all around that come through fine and nothing will remove it. Is the tv fixable? Am I accidently pressing the wrong button on my remote? Its rather annoying! The brand of my TV is Advent purchased from Meijer in Michigan, not sure if this is brand under another brand. This is a 25 inch flatscreen with a builtin dvd player.