Why won't my music show on my sansa?

I have a sansa e200. Ever since i’ve had my sansa, i’ve dragged and dropped my music into the media player, then would hit the sync button to transfer my music. It had worked for me up until the other day, but i have no idea what happened.

I had unplugged it once what i had on my list of music had transferd and it was all there so, i plugged it back in to add more. Once i had completly finished, i went to listen to my music, but it showed it was empty. There was no music yet, when i went under “my computer” and clicked the file for my sansa my music was all listed there and it said it was all on my device…