Playlists disappeared from Apple Music application on MacOS X

I have found zillions of questions on playlists disappearing on iPhone, this is not my case: my playlists disappeared from the Apple Music application on my Mac.

I also found zillions of questions on how to restore playlists on iTunes for Mac. iTunes does not exist anymore, instead I have an application called Music.

Suddenly, without any update or upgrade or other change in my system, playlists I have been building for ever since MacOS X exists (20 years?) have disappeared. All "songs" are on the disk, it has nothing to do with iCloud. Most of the songs have not been downloaded through iTunes and an Apple account but just have been dropped on iTunes (or now, the Music application) and have been thereby copied on disk. The songs are physically there on my disk (140 GB) but Music sees only a few of them (probably those that have been bought through iTunes and not those copied from other sources) and not a single playlist.

How can this happen? What can I do? I have an Arq Cloud backup, but what files should I restore?

Thanks in advance. This is a nightmare, more than a hundred playlists have vanished…