Black box in middle of tv screen

This is a background for text, even though you aren't seeing any text at the time. Since it covers most of the screen, it sounds more like a demo mode screen than closed captioning. Go into your setup menu and make sure that neither of those is selected. If necessary, restore the factory settings. I'm assuming that this is coming from the tv itself, and that you don't have a VCR, Tivo, etc. Good luck!