Problem with my Sansa plus and switching computers?

I have a 8gb Sandisk Sansa clip plus.Recently I switched from windows to Ubuntu and though the player is full with music,the folder (MSC mode) is empty on the computer! I assume that’s because I used windows/windows media player and MTP mode.When I transfer new music,It appears normally both on the player and the computer.But the old music that plays normally on the player - seems to be gone on the computer.what can I do to fix it?!

Thank you