I see that this message in unanswered since several days so I will attempt to reply.
First, disconnect the TV from the mains and let it rest for a few minutes. This will reset the processors. Then, try to connect another source to the TV in the VGA input, such as a computer. Check also the set top box on another TV, as it may be defective.
You will be then able to discover the defective component, the box or the TV.
By the way, is your TV connected to a Beosystem (1,2 or 3) or you are using the Beovision 4 as a standalone TV ?
Good luck,
Jean