iTunes Libraries and playlists disappeared

I use a Apple Airport to store my music. The music was bought in the store or imported from CD's during ten or more years. I don't use Apple Music. I have a MacBook, OS 10.14.6. Since a couple of days my music library is empty and my playlists are gone! Only the albums I bought are still visible and the rest is nowhere to be found in the iTunes app. The airport data disk is accessible, the music is still available, but the connection between iTunes app and the music library seems to be missing. How do I rebuild the library (if that is the trouble) and how do I get my playlists back?