Hi dita97,
Thanks for using Apple Support Communities! I see you're having some trouble with your Apple Music library, and I'd like to do what I can to help out.
I'd like to clarify, when you say your music is missing, are you referring to music downloaded to play offline?
If so, if you logged out of your Apple ID used with iTunes & App Store, then logged back in, that action will clear any downloaded music. You should see the cloud with the down arrow next to the song to redownload those songs for offline listening. You can still stream those songs.
Are you referring to your music on another device and you are using Sync Library to sync your library to other devices?
Turn on Sync Library with Apple Music
Can you please provide some additional information? Such as, how the music was added to your device, Sync Library being used?
Do you see the download cloud symbol next to the songs?
Thanks!