A playlist is simply a file with a list of songs that points to the location of the songs. A playlist contains no music. It is not an audio file. So if you move the location of the songs after you created the playlist, the playlist won't work anymore.

Answer: Move the songs back to the same location they were at the time the playlist was created.

Else don't worry about the playlists & just think about the songs & create new playlists. WMP is a playlist player. It is simply a reference player. You can re-add songs to WMP's playlist(s) by selecting the WMP media library tab, then press the Add button at the top & direct it to where the songs are.

iTunes is more durable b/c it has its own music library. You can re-add songs easily to the iTunes music library. See How to transfer music to my iPod (& iTunes) Of course you can create a playlist within iTunes & open that playlist within WMP (AFAIK.) I think you need to export the playlist file. I could be wrong about that point.

* I hope you saved all the songs that were in the iTunes Music Library. You did save all the songs didn't you & not just the playlists?