You haven’t said what player you have or the method you’ve used in putting the music on, so it’s hard to guess what your problem may be. We’re not mind readers, you know.
Generally speaking though, it could be your song files have ID3 tags resulting in everything be classified on they player as ‘Unknown’.
Or it could simply be that you are trying to transfer an un-supported format. In other words, a music format that is not recognized and not playable on the Sansas.
There are a number of other possibilities, but knowing virtually nothing about your particular situation, I’m not going to list them all here.
Be very specific in your reply and we will try to help you, or feel free to browse or search the forum for answers. Chances are somebody else has had the same issue. There are very few stories we haven’t heard here. There are boards here for each individual Sansa player (or series of players).
The ball is officially in your court.