I'm not really sure that I understand your question but if your songs disappaerd you can try to search for your favorite single, album or artist and save the songs you like to a playlist (this can be a whole album or all songs of an artist if you like).
This makes it also easier to make some playlists availible offline and others not.
Go into the Spotify settings and make sure show unavailable songs is turned on under playback.
The reason is that the artist or their music label has likely decided to remove it from Spotify.
The availability of music on Spotify is up to the artist and their music label.
Spotify wants all the world's music, and they're working hard on making this happen for you. However sometimes agreements can't be reached with the artist or label or a change may happen in music ownership. Sometimes the music is only available in certain countries, while access is blocked to others. Sometimes the album or song will be unavailable temporary, and come back again later.
Spotify adds thousands of albums and artists are added every day. If you can't find an artist you're looking for, they may appear (or reappear if removed) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.