Soundiiz manages your music service accounts directly through the API. That means all actions made on Soundiiz will be done immediately on the targeted music service.
For legal reasons, Soundiiz is not creating any backup data for third-party music services connected.
We recommend you contact the music service where the data has been deleted to see if there's a way to retrieve your playlists/favorites.
Some music services have a page for that, like Spotify and Deezer.