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The problem has to be internal in the control head in some way. This system is electrical and vacuum operated so there wouldn't be a cable or anything that could come into play here.
Outside of trying to force it to move the only thing you can do is try removing the unit and taking it apart and see if you can determine what happened. Something internal in the control head has to have broken or the mechanism is seized. You may be able to disassemble the control head and free it up but you may end up having to replace it. If any internal parts are broken beyond use or you are unable to get it freed up then the control head will need to be replaced.