Check your HDMI connections
Your HDMI cable might be acting up. Disconnect it from your YouView box and TV and then reconnect it.
Still not working? Try using a different HDMI cable.
Restart your box
The old “switch it on and off” routine. Overused but it can often help resolve the issue. Press and hold the 
button on your box for 8 seconds, then let go. You’ll be greeted with a friendly ‘Hello’ telling you that your box has restarted.
Further help
We didn’t think you’d end up here, but thanks for trying all the above steps. We’d suggest contacting your box provider for further help.