there are direct mechanical linkages with the controls, so its like losing your power assist in a car, and its severity depends on the type of machine. a jetranger can be flown hydraulics off fairly easily down to quite low airspeeds, whereas a multi-bladed system with heavier disc loading is much more of a handful (e.g. the as350). i don't fly astars myself but i believe they have accumulators to give the pilot a limited amount of time with hydraulic assist after the system fails to get the helicopter slowed to a more manageable speed.