The same way you steer a car with normal power steering when the engine dies: you pull harder on the steering wheel. In both cases, you have power assisted steering, there's still a mechanical connection to the front wheels.
Actually, I would expect the Prius to be easier to steer without the power assist than normal cars. With the electric assist, if it shuts down, you should just be steering without the assist. In a nornal car, you have to fight against the power steering pump, and I know from experience that it's harder to steer than if you had the same car without power steering. It's not that horribly difficult, but it is more work than I'd want to do all the time.