If it is closed captioned through your tv, I do agree that is what you could be seeing.
Most tv's have 2 closed caption settings. On mine they are Caption which has the words at the bottom of the screen.
The other setting is Text, which has only a big blank black box on the screen.
I don't know if these settings are standard on all tv's. Yours may say something different.
Do as Nick suggests and try the tv CC settings first.
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So, please explain how CLOSED CAPTION setting was changed. There has been no adjustments made on our box???
Try going into your TV set menu and turning off the closed captioning option. You will need your TV set manual and-or your TV remote control and locate the CC instructions. Buttons on your TV remote control may have been pressed accidentally.
If you have a HD cable box, then you can also try turning off any closed captioning on the box too
Turn the cable box off, and leave the TV on. Then press the Menu button on your remote control, it should bring up the HD User Settings menu. You will be able to disable closed captioning there.