Black Box on the screen

It sounds like a text box that isn't receiving any text. If it covers almost the entire screen, then you might have accidentally got the settings into DEMO mode. It could also be a box for closed captioning, but that usually covers just the lower part of the screen. Either way, the solution is to go into the setup menu and restore facotry presets.

Try pressing the "Cap/Text" button on your remote until it cycles to the "Off" selection of it's three options:
(1) Off
(2) Caption 1
(3) Text 1

It could be that that black box is a text box without any text available. So turning this off should make the box disappear.