Although the Apple TV box could find the media library shared from the Mac it refused to play any videos. it had worked earlier in the day. A new file added to the library would appear in the library on the TV but would not play. They were all signed in to the same Apple account and were on the same WiFi network. What to do?
Restarting the Mac did not help.
Unmounting and remounting the external drive where the library lives did not help.
Restarting the Apple TV box did not help.
Signing out of and back into the Apple account did not help.
Restarting the WiFi router solved the problem and all was working again.
🤷♂️