I am a premium Spotify user in the UK and whilst travelling overseas last week I couldn't access any of my playlists even though they were made available offline. I understand you may need to establish an internet connection to be able to access the music however when I got on wi fi it started to download all my music again.
By any chance you're using an iOS device and it's storage is almost full?
When iOS devices are running out of storage, the system automatically cleans the data from the apps, in a desperate attempt to get more available space. Included on the files the system erases, are the Spotify files, which are the stored tracks the app downloaded for offline play.
We can't be sure that's the cause of the problem because we don't know what's your device and OS, however.