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You probably turned on your Subtitle / Text setting, and your DVD disc activated it. Follow this information --
You may have to test it on another TV set and make sure the DVD player is creating the black overlay, and not the TV you're presently using. Subtitle systems have been available since the mid-80's on TVs and Dvds. Parental lockout is possible, but all you need to verify that is a kid cartoon/movie to know its your problem. I always look at the LAST remote that was touched just before your problem started. That's likely to have a subtitle/text button that got triggered.