That's weird. If you can see the guide, then clearly it is connected up to your display device but I wonder if the output settings are configured correctly for the normal picture. Possibly there is a problem with the box making it unable to present the TV picture.
Tell us more about the freeview box, what it is connected to and how it is connected?
I would be tempted thought to try simply plugging it into another display. Have you tried another Freeview box? Given you can pick them up for less than 20 quid these days, got to be worth trying.