Are you using a TalkTalk router supplied within the last few years, and is your broadband download speed greater than 5Mbps? Is the router connected directly to the YouView box by ethernet cable?
Can you switch your router and youview box off at the mains for a few moments, then switch the router on and wait for it to reconnect to the internet. Switch the YouView box back on after the internet comes back, immediately pressing and holding the plus and minus keys on the front of the unit. Use the service menu to do a factory reset and hopefully after it sets itself back up you will have these channels.