If you're referring to a lost playlist, you can recover it following these steps.
However, if you're referring to the Songs section of Your Library, they can't be recovered at the moment. If this is the case, let me know if someone else has access to your account, or if you've seen other unwanted changes, like new playlists or songs you don't recognize.
All the best.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp