music.app not finding my local music library

I've had a music library on my mac since the earliest version of iTunes. I keep my entire library locally on an external drive and don't use iCloud or match or any sort of sharing. I took my laptop on a trip and while away, never had cause to hook up my music drive and start the music app.

Now that I'm back home, I've hooked my drive back up, and I can see that all of the files are as they should be, but the music app insists that my library is empty and sees none of my playlists, played data or songs.

How do I fix this?