I have the same problem, last year there was a loud grinding coming from the antenna but it still moved. Now it just makes a 'whirring' sound and does not move at all. Seems the power antena motor is OK but a gear has finally moved off track or been ground down to nothing.

At least mine is stuck in the up position.....so it doesn't really bother me...

I have heard of people replacing this motor, or just buying a rear window antenna as a replacement.