Black box in middle of screen

Within seconds of turning on the TV, a black solid box appears in the middle of the screen. The black box does not cover the entire picture and you can see the picture around the outsides of the black box. If I hit a number or volume button, it disappears for a few seconds then comes back. I have DVD player hooked up to the TV and I can play a certain DVD and the box won't show up at all. Any suggestions?
Thank you.
Tammy