Do you have any problems with other apps or is it just You Tube?
Have you looked at Check service status at the top of the page, it lists any local faults?
You can also a run a test on your equipment from there. It may send a signal to your box to rectify the issue, and in some cases can automatically order a replacement box or engineer visit.
You should also check all your cabling is finger tight, especially the white coaxial cable.
If that fails then you need to contact the Faults team.
You could try the text messaging service. Just send a text with a description of TV Faults to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible.It may take a few days.
You can also call 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 2 (Faults) butbest to call at 8am to avoid call queues
When calling Faults you will go through a series of questions/tests, if that doesn't resolve the problem then call back around 10 minutes later and an agent should pick the phone up.
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali