Could your closed captioning be turned on? I have 2 of these boxes, they have a dedicated "CC" button on the remote. It's in the lower right of the number pad, the "-" button. Just press it until it displays "captions Off" to turn off the closed captions.
I suspect someone was playing with your settings and turned it on, and possibly set it to a service that has no data, or the text and background color have been set to black, thus it just shows just the black box. Go into your menu and reset the closed caption options (#4) to "default".