The film advance lever won't advance because the shutter has not completed its travel to be ready to be retensioned. If someone has tried to force things in the past, well I am sorry, but that can make things helpless.
This may sound loopy, but a first try is to put the thing in a cool to warm oven (say 150F). Let it bake for few hours, sitting on an oven mitt, so not one hot metal part of the rack can perhaps overly soften any part of the camera.
This can work to soften any old oils that may be one of the reasons the shutter is stuck.
Also see if there is a self timer. if it perhaps has gotten thick oil near it, and wont finish its travel without a physical nudge, well that can be a reason that the shutter will not finish its travel.
Good luck. A nice looking manual range finder. I had not been exposed to this unit before.
I did find a reference on another page to a fix listed on the nelsonfoto forum. I am not a member there, but you may wish to register there and sniff around.
The next trick is to get the
