Welcome to this user forum.
Have you tried to see if there is a software update for the app via the Samsung TV menu?
Can you access Netflix on a computer?
So is it the BT YouView box that you have, and the Netflix app on the YouView box that does not load?
Is this a new YouView box that you have just received?
Has the YouView box been connected to the Internet all of the time, as there have been a number of software updates which may have updated the app?
Have you tried turning off the YouView box from the mains, then letting it start up from the beginning, as that will force a software update if one is pending?