Could it be that the music you are trying to play is an M4A file (the file type used by Itunes) and not MP3 (generic file type used by most music players)?
If the files are M4A, then they will only be accessible through iTunes.
If that is the case, and you want to use the tracks through WMP, then you can use iTunes to convert the files to MP3.
Sorry about the brevity of the details .The files are mp3 and the message displayed is Player may not support file type or support codec used to compress files. I have the same problem with CDs that ihave put on the computer ie USED SONGS -GYPSY SONGMAN that have previously played alright.