Playlists in Apple Music

I just opened Music on my Mac and my library is gone. Not the physical media, just their existence in Music, along with a few hundred playlists. It looks like it's completely reset to factory settings.

Since the change from iTunes, I have no idea where all of that data is stored, since the "iTunes Library" is basically obsolete. My question is, where are they stored, and can I get them back?

I should note that all of my media is on an external USB drive that normally stays docked permanently to my iMac, but just today I ejected the drive. It would seem that would have something to do with it, wouldn't it? Thanks for any advice.