What happened to my music?

I own a iPhone 8 Plus with the 11.2.6 OS version. Recently my App Store wouldn’t allow me to download any apps, I was advised to log out of my Apple ID and log back in and this should help. I did and it worked! But now all the music I had downloaded and all my playlists are gone and I was wondering how do I get them back without having to add them all again one by one? And why did they disappear?