Same. Got my new TV yesterday. Used Prime and all good. Went to use today and wouldn't work. Cleared the cache. Did a hub reset and now it won't install the prime app.
I have the same issue
Couldn't open AppleTV app. After trying unplugging the TV and back on, I reset the Smart Hub. Frustrating to have lost all my apps and need to log in again. In the end this did not fix the issue. Obviously something has happened for so many people at the same time to have this issue. If there was a firmware release in the last 24 hours, then it is most likely that. If not, then there must be a server issue. Would be nice for Samsung to acknowledge that their customers are experiencing an issue they are most likely at fault for and are actively trying to resolve.