Lots of troubleshooting possibilties.
First, try someone else’s computer, so you can see if it’s the unit or your system.
Could be a power problem. You need to connect directly to the computer–no USB hub. A back port is better than one in front, to provide more power.
Could be software annoyances. Try:
Settings/System Settings/USB Mode and change to MSC. That’s the generic USB connection and the one that works most often.
Still no love? With the unit connected, go to Device Manager (look for the method in your Help files) and see if it shows a yellow triangle with an exclamation point under USB ports or Unknown Device. Right-click on that, Uninstall it (not disable), reboot and reconnect. That would fix a Windows driver problem.
I don’t know what Windows you are using but in some older systems installing the MTP Porting Kit (from support.microsoft.com) magically solves this problem.
And it could be something as simple as a bad cord. To test that, you’d have to try another cord. Know anyone else with a Sansa? Or a store nearby? Don’t use an iPod cord–they are wired differently and will fry the Sansa.
If none of these helps, call SanDisk and get a replacement.