I just tested this with MC 20 so it should work, but be careful and think about the steps. Here's how I did it:
1. Make a backup of your library with a name that indicates to you that it's today's backup. You're going to need this backup later in the procedure.
2. After the backup finishes, go to Library -> Restore Library.
2. A. Find the older backup that contains the Play List that you want to get back.
2. B. Choose that backup and restore it.
3. Your library should look like the old one, and you should be able to find the Play List you want. Select the Play List and choose Export Playlist. Export it as MPL format (it should say "Media Center Only" next to it.). Choose a name and location for this exported Play List that you can find easily. You'll need it in a minute.
4. Now that the Play List is exported, you want to switch back to your current library. Choose Library -> Restore Library again. This time, find the backup you made in step #1 above. Choose it and press OK. Once it's done, your library should look exactly like it looked before you started at step #1.
5. Go to File -> Import Playlist. Go find the Play List you exported in step #3. Select it and press "Open".
6. This older Play List you were looking for should now be under Playlists -> Imported Playlists . Go find it.
7. You can now rename it if you want to. You can also drag it out of the Imported Playlists group into the main Playlists area.
That should do it. Good luck and let me know how it works out for you.
Brian.