Hi ianastronomy,
As s130 rightly said, it is 99% sure that the toothed belt that pulls and pushes the aerial has gotten brittle and snapped. I replaced mine with a generic unit that has held out well. It is a little taller than the original aerial, and the look of it on the car is a little odd, but it does the job. I think it was about 20-30 pounds, so not too earth-shatteringly expensive. I personally didn't bother with repairing the belt because it is just too much hassle (if you do decide to open it up you can see for yourself).
Give me a few days to look for it on the Ebay. I bought it about a year ago so it might be a little hard to find.
Thanks,
Chris