Mine have always been hard to move, but I haven't ever BMW power locks totally frozen up. I wonder if the central locking module is energizing them closed? Sometimes the little door lock micro switches lock up in interesting positions, maybe that's the problem? Any difference between key on or off?
If you get a chance, pull the central lock module (it's in the driver kick panel IIRC) and see if the locks will manually work.
Could also be a simpler problem - maybe the door mechanisms just need lubed up?