Check in the Default Programs section on your "Start Menu" perhaps Microsofts media player has taken control of your mp3 files and iTunes is no longer the default player for those type of files. Another thing to try to make sure what player is assigned to which fiiles is to right click on an mp3 file and select "Play" if they come up in Media player then that is what has happened. It is an easy job to go into the same section and reset those mp3's back to open with iTunes.