Restart your TV and when the TV turns back on, then restart your YouView box
Remove then re-insert your HDMI cable from both ends at your YouView box and TV, or AV Amplifier if you are using one
If you have a SMART TV please make sure that the TV software is up to date
If you’re connecting via an HDMI switch or AV amplifier make sure that the software for this is up to date. If it still fails try connecting directly to the TV
If you still have the same problem, try a different HDMI cable and an alternative HDMI input in your TV