Could it be that you moved countries?
Apple’s media Store for movies and TV shows only sells TV shows in 6 countries (US, CA, UK, FR, DE, AU).
Setting your Apple Account to a different country would invalidate some or all of the purchases from the former country.
Buy movies and TV shows from the Apple TV app - Apple Support
Availability of Apple Media Services (by country) - Apple Support
About changing your Apple Account country or region - Apple Support
Watched TV-show content from compatible connected apps would appear in the TV app (e.g. Continue Watching). Searching new shows inside the TV app from these content providers may be limited, though. Better initiate that in the external content provider app for their full catalog. Once watched a bit, it would then appear in Continue Watching.
Availability of supported apps in the Apple TV app (by country) - Apple Support
This latter part applies to Apple devices like iPhone, iPad, Apple TV box. It does not apply to third party devices like Fire TV. The Apple TV app on third party devices doesn’t get the ability to connect to external apps, as all apps have this limited through software sandboxing as a security feature.