Could be many things.
Have you had an update for your music player recently?
It may be that the playlists are no longer recognised or they may have been corrupted - do the songs play outside of the playlist or maybe through a File Manager?
Alternatively, it could be that the music files themselves have become corrupt - can you try to play them via your PC? Connect to the PC using a USB cable, find the files in Windows Explorer and double click the files.
If none of this helps, try re-posting with a details of your phone and music player.