I don't know exact setup on the Massey, not sure if it helps but this is what I would generally look for on most equipment.
I don't think there would be a solenoid to open system.
If other hydraulics like header lift work then pump should be okay. (assuming the steering doesn't have it's own pump)
Try lifting head and steer at same time. My L2 gleaner has a gear pump with open center flow back to tank. Raising head may restrict flow a bit and give you some steering if priority valve is the problem. If you have variable flow hydraulics it may do something similar by increasing flow.
Next step is a priority valve. This supplies oil flow to steering first then other hydraulics. You could have a broken spring or sticky valve.
After that is the valve attached to the steering wheel. I haven't had a sudden failure but it's possible to have an air lock. Have you had a leaky line or hose off recently?
Sorry can't offer specific advice. Let us know what you find out.
Could you try taking the steering cylinder off on the back axle and see if it moves in and out freely, possibly the packing has let go? Maybe remove the oil lines first and see what happens? Where are you from? Is it below freezing? Water froze somewhere in the system?
If it just went from working normally to nothing while sitting in the shop you have to suspect something simple like a stuck priority valve (open center system) or air rising to the gerotor under the steering wheel. When you say you can turn the rear wheels with some "horsepower" on the steering wheel you must have oil in that part of the system. Try to determine if the steering wheel is putting pressure to the steering ram as some others suggested. Will not be a ram piston seal if you can turn the rear wheels by force on the steering wheel. Process of elimination!!
there is a bold (cap) on the bottom of the priority valve that holds the piston and spring in place when we took the bold off we could feel the
bottom of the piston but it would not come out ( we had previously hit the valve with a hammer hoping to loosen the piston ,no luck )
then we took of all the lines and on the work bench we checked it over , reinstalled everything and all was good