Open the Device Manager on your PC, with the Sansa plugged in. Do you see the “connected” icon on the Sansa’s display?
The Sansa will be listed under Portable Device (if in MTP) or under the USB Root Hub if in MSC. Right click on the Sansa and select uninstall. Unplug,wait 30 seconds (I prefer to reboot the PC at this point), then plug in again.
See if the PC now lists the Sansa in Windows Explorer (My Computer).