Have you rescanned your library since re-adding the file?
If you have and that didn’t bring back the file immediately, try out these steps (aka Plex Dance):
-
Explorer
→ move the file outside the scope of Plex -
Plex
→ scan the library for file updates -
Plex
→ empty trash, clean bundles -
Explorer
→ return the file to its original place -
Plex
→ scan the library for file updates