I just had the same thing happen.
The electric bits are fine and based on the noise the motors are still working, so it'll be the worm gear arrangement that has gone.
The advice I got was to go in from the top, removing the plastic seal on the roof, then apparently there are just a few bolts, which allow you to remove the glass and get in.
I haven't had a go yet, my thoughts are to just get it shut and secure first.
nick
July 2008