Welcome to the forum. That sounds like a HDCP issue between your box and TV that can happen occasionally on anything you watch. You may well find that it's just sorted itself out the next time you try but if not have a look at the following help page which explains what is happening
It's a case of messing around with the HDMI cable and TV until it works .... Start off by unplugging both TV and box at the mains, unplugging both ends of the HDMI cable and reconnect, then plug box and TV back in and see if it works.
