This is a common problem that some people have more that others. I have two V6 boxes and in two years I've only seen this 3 times.
Suggestion is that it's something to do with the HDMI connection and the copy protection system.
What type of TV do you have, HD or UHD (I assume you haven't just changed it).
Suggested fixes are.
1. Change HDMI settings on the V6 box - try 1080p + passthrough
2. Reboot the box.
3. De-register and re-register the amazon account to the box (via the amazon web site) - reboot the V6 after the de-registration