Your computer can only ‘see’ files that were added in the mode in which it is connected. In other words, if all your musci was transferred to your player in MTP mode, you will not be able to see or access those files on your computer unless the player is in MTP mode. In MSC mode, you’ll see nothing. The songs are still there; you just can’t see them.
Likewise, if your music files were transferred in MSC mode, the player has to be hooked up in MSC mode before you can see or do anything with them. The problem with the Auto setting is thaton can connect in either mode without your knowledge. so if you use multiple computers and it’s connectiing differently on each one, it can make it real confusing.
So, go into Settings > System Settings > USB Mode and change to the other setting, re-connect and you should be able to see your music.