Hi.....Years ago I had a 650G. It had power steering but that was really controlled buy a hyd. cyl. mounted in place of the steering link. So I think the steering box was the same, mounted down at the end of the steering column.
Anywho, I could not turn the steering wheel after sitting outside in the winter. I heated up the gear box with a bernzamatic to get it inside. I then drilled a hole, threaded and put a pipe plug in it. Added new oil and worked fine.
Yes yours is probably froze, at least my was. George