Exact Model Number of the TV would be handy so any potential respondents can look up a manual and/or go by personal experience in trying to assist you.
Anyways ... a few things to try:
Ensured that the TV is in fact connected to the access point/Internet?
Tried resetting/updating/uninstalling & reinstalling the YouTube app?
Have you ensured that the time/date/location settings on the TV are configured correctly?
Note: YouTube has (and will probably continue to) discontinued app updates for several TV's - try watching through the TV's browser if all else fails.