Soundiiz is managing directly your music service accounts threw 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 datas for third party music services connected.
We recommend you to contact directly the music service where the datas 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.