...so far pointing to a failure in the HDMI cable (check by substitution) or the box.
The Soft reset of the TV Box often works to fix strange operating faults. Press and hold the YouView front panel power / standby button for 8 seconds, until the lights change then let go. Then just wait for the box to restart. Or, when the hard drive isn't active do a Power reset. Power off at the back on/off switch or mains power socket, wait a few minutes and repower.
More complex issues might require a Maintenance Mode reset of the TV Box (Click here). Read the instructions so you know what to expect, select your box model and when prompted on screen select Option 2 (Keep recordings) as a first choice.
If the Maintenance Mode reset works then restore the box settings and at the same time it's worth rebooting the router and any connecting powerline adaptors and check they've got green lights showing.
You'll need to reinstate a lot of personal settings, including parental control PIN if used, Antenna out back to On, Picture & Sound setup, Apps & player logins, Mobile devices reconnected for the TV Planner app and TV Guide (yellow button) hidden channels.
Any Watch lists and Scheduled recordings will need to be reinstated so make a note of these before a Maintenance Mode reset.
I've alerted the TalkTalk Community Team. They'll respond here probably with the offer of exchanging the TV Box so long as you're still subscribing to a TV Boost.
Looks like your Community Profile Personal Information (Click here) includes your name, address, current TalkTalk landline 'phone number and an alternate number (mobile recommended). Check, amend if needed, then scroll down and save changes. The TalkTalk Community Team OCE will then be able to link to your TalkTalk service account.
I'm sorry to hear this.
Some TV’s may use a later version of HDMI, can you check if you have an option within the TV Setup Menu to change the HDMI version?
There is also a compatibility issue between some TV’s that use Deep Colour Mode and non UHD Devices such as the youview box.
Please can you check the TV settings menu. On some TV’s they may need to be in AV mode rather than TV mode. Can you also check if there is an option to turn UHD Deep Colour Off?