Empty playlists in Music app on iMac running Sequoia 15.4.1

On my desktop Mac, running Sequoia 15.4.1, the playlists I created in iTunes (now Music) are suddenly empty. "Smart Playlists" display songs and they play fine, but playlists other than "smart" show as empty. How can I fix this?

[Re-Titled by Moderator]

Original Title: Playlists suddenly empty