Sandisk Sansa players have had a bit of a history hiding some files away before. I believe if you were to disconnect your m230 and turn it on, you would still have that music available. If that is the case, it is most likely a problem with the USB transfer protocol.
If your music is still on there and can be played, there is actually nothing to stop you from transferring more songs onto your device. Once the files are loaded and the device disconnected, your m230 should update accordingly. The drawback is you cannot find and remove your old songs from your computer.
If that is what you want, try this:
With your Sansa disconnected from your computer, turn it on and navigate to the Settings Menu. Then select the USB submenu. There should be two options: Auto Detect and MSC. If one is selected, try the other and reconnect it to the computer. Check the folders and see if there is anything in them.
Worst case scenario is there was a glitch in one of the USB modes. A reformat, either by restoring factory defaults on your device or using your computer to reformat should start you from scratch.
I would recommend reading the manual for any other questions you may have. As with an increasing number of products, manuals are posted online by the manufacturers.
Sansa m200 series manualHope this helps.