I was scratching my head I discovered my playlists disappeared on my Samsung Galaxy S4. I wasn’t used to the quirks of my S4 yet because I only had it for a few days. The strange part was that I was listening to one of them.
To correct this problem, you just need to update your library. I store all of my music on an external micro SD card. I added a few files the night before by connecting the phone via USB cord. Once you updated the library, the playlist should be restored.