Cant see my music

There are two USB modes you can transfer files in: MTP and MSC (there’s a third option on Sansa players, “Auto-detect”, but that just automatically selects either MTP or MSC depending on whether working MTP drivers are present on your PC. MTP is part of Windows Media Player 10 and up). Files loaded in one mode can’t be seen by your PC while connected in the other mode, but your Sansa will see all files loaded in either mode. This is normal. You probably loaded files in one mode and are now connected in the other, possibly because “auto-de f ect” (as it’s commonly known around here) decided to change modes for some reason.