My Zune would not sync, but the battery would charge. I checked the USB driver for the Zune (listed as Zune in device properties) and it was corrupted. I uninstalled the device driver then plugged in the Zune. The driver reinstallled and the Zune works properly.

Right click on My Computer (or Computer in Vista)>Properties>Click on device manager>find Zune and see if there is a yellow exclamation point. If the exclamation point is there, the driver is not working correctly.