Lost my iTunes playlists after update: how to get them back?

The default location for a Music library in Catalina and later is ~/Music/Music with the media files consolidated into ~/Music/Music/Media. In Sierra I believe it was ~/iTunes/Music and ~/iTunes/Music/Media.. You might look there and see if you can find the playlists and export and then import them to your new library.

Save a copy of a playlist in Music on Mac - Apple Support

If they were smart playlists, you can try rebuilding them using this general procedure (thanks to tt2):

Restore Original Smart Playlists - Apple Community

Hold down option as you launch Music, when asked to choose or create a library click choose, then browse to your old iTunes Library.itl file, then pick a location to save the convert Music library. You might want to rename ~/Music/Music first so your converted library can go at this default path.

tt2

You're welcome. 🙂

tt2