Do you use Time Machine or similar to backup your computer and important data? If not now is the time to start.
When the Music app is updated as part of an update to macOS it is possible that there will also be an update to the internal layout of the .musiclibrary package. When this happens the existing library is backed up in the Previous Libraries folder in your ~/Music/Music folder and used to make the new version of the library. On rare occasions this process doesn't complete correctly leaving a corrupted library that may be missing some of the expected content, playlists, etc. Assuming that is the case here copy the most recent package from the Previous Libraries folder into the main ~/Music/Music folder, rename the package Music Library as, say, Music Library (Bad) and then remove the date from the package that you copied in so it becomes Music Library. Lastly hold down Alt/Option as you launch Music and use the Choose Library... button to browse to an open your library at ~/Music/Music/Music Library. If all goes well you will now see your lost playlists.
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