The steering control unit at the base of the steering column contains a pump so even with the engine off, you should be able to steer your tractor, but with additional effort. I think your pump shaft has come loose from the steering column or perhaps the pump drive shaft has broken. This is the pump in the control head, not the hydraulic power steering pump. If your power steering pump was not operating, you would simply have to work really hard to steer. Since yours spins freely, it sounds like a broken control valve pump drive shaft or something similar. I have never taken the control head apart, so I don't know how big a job it is.