TV Repair Questions? Ask a TV Technician for Answers ASAP
What's wrong is that you have accidentally turned on one of the closed caption options called "Text." This function of closed caption is practically never used. If it were operational, there would be text inside this black box.
Just go into the TV's menu, look at the Audio options, and turn Close Caption and/or Text off. That should fix it.
Hope that helps, thanks for using JustAnswer!