I've read and re-read the Wiki on Libraries, and I think I have it, but I clearly don't. I'm a music-only user, and I have all my music in one library. I use a PC to run the MC client, and I'm attached to an Id NUC as my server. I created a new Library, but because I'm new to this function, I immediately tried to return to my main Library with File -> Library, and choosing it from the list (just to make sure I can switch between them as easily as some folks say), and something is messed up.
My library is called bytor. When I tried to select bytor from the File -> Library menu, the client reports, "There are no files. Double-click here to add files to the library." Yikes!
Going back to the File -> Library menu, I see not one, but TWO libraries in the list called "Main Library", and both behave identically: they show all of my music files, but trying to play anything results in an error message saying, "Something went wrong with playback. Details: The file could not be found. Check that the filename in the Library matches the filename on disk." DOUBLE YIKES!
So, I quickly deleted the new (completely empty, untouched) Library I created, hoping that would somehow restore things to where they were just moments before, but alas no. I've rebooted the Id, I've restarted the Client... no luck.
Interestingly, I'm able to play my music no problem through JRemote (on my phone), and using JRemote as a controller for the Id on my home stereo, so this is a Client issue. And that's how smart I am.