The plastic toothed belt that is cast inside the antenna has snapped and the motor has nothing to grab on to and move.

If you can find a replacement antenna mast, all you'll need to do is remove the trunk panel and the driver's side rear interior panel to access the antenna motor body.

Be careful not to damage the paint when you remove the chromed locking nut. Masking tape and vise-grips will help immensely.