Did you import the actual songs to WMP just import a playlist. A playlist is simply a reference to the location of the files. It is not the actual songs themselves. So if you move or delete the songs from the location the playlist was created then the playlist won't play. That's why I use iTunes b/c it physically imports the music to its own music library.

I have a suspicion you deleted your song files. I'm not a knowledge on WMP. It sounds like you were sharing your music library & deleted it thru the LW library.