greetings everybody!

i'm a new Sansa user (almost 48 hours now  ), and i'm more than happy, but this strange thing happened today and i don't know if i did something wrong. all albums on my computer have their own playlists, and i've draged and dropped some music on my Sansa last night. i've tried to play playlists, and everything was ok. when i select a certain list, a list of songs appear. but this morning all playlists were empty. all of them are still recognized by Sansa, and all the music is there, but when i try to select playlist i get the message that it is empty.

did i do something wrong?