Picture Problems. After we have our TV on for a short while, a solid black box pops up on top of

Question

After we have our TV on for a short while, a solid black box pops up on top of the picture. The box covers the middle half of the screen and has a series of different letters inside the box. The picture continues to be visable and the only way to rid the picture of the box is by turning the tv on and off. This works but we may have to turn the tv on and off several times a night. What is this and how do we get rid of it?