Help!! Suddenly , Plex Home Theater won't. recognize the properly formatted mkv files...

...on my external 6TB drive - despite the fact that all of the library data is all there (and correct)
When I click the play button on a movie, I keep getting the message "Unavailable - please check that your media library is connected and available"
This is the same hard drive connected in the same way (thunderbolt) that I have always used.
2013 27" iMac, OSX 10.11.6, 16gig of ram