Sudden Loss of power steering whilst driving. No symptoms, noises, etc. before hand. Power steering fluid level is normal.

Hi Lisa, I'm not certain, but it is possible that the power steering has a separate belt. If the fluid level has remained constant and there are no noticeable leakages, a broken belt is the most likely cause. Regards John

Have you bled all air out of the system after refit?
Check the oil level and for any signs of air in the oil.

To bleed the system, you must turn the wheel from lock to lock, whilst topping up the power steering fluid. After a dozen turns or so, stop the engine, and allow the air in the fluid to rise out over several minutes.
Restart the engine, and work from lock to lock another dozen times or so, checking the oil level again.

After doing all that, is there still air in your system?
Is the power steering drive belt slack?

If everything is normal and operating properly, then you may have an issue with your steering rack.

Hi
If your system has leaked fluid out when you've topped it back up it could have caused an airlock in the system this can usually be rectified by turning the steering wheel from lock to lock several times whilst the vehicle is stationary this usually pushes the air out but keep checking the fluid bottle as the level will drop whilst its getting rid of any air !
Hope this helps Nick

Check your fluid level first. If the fluid is low it will cause the power steering pump to make noise. Moore noticeable when in parking lots and driving in the city where you are making a lot of turns. Look for leaks. If you fill the power steering resivor to the proper level and the fluid leaks and you can not see the leak it may be your steering rack. The fluid leaks into the boots and sometimes goes un -detected. You can poke a small hole in the bottom of the boots and see if fluid leaks out. If so the steering rack is leaking and should be replaced

hi Eva.
As you turn your steering wheel it is this fluid pressure that aids in turning the wheels of your car. ... If the pump impeller is trying to move power steering fluid and starts catching air bubbles it can cause whining noises and vibrations to occur.
Maybe you've heard it at a stop light. Maybe you've heard it as you pass a car on the freeway. Or maybe you've heard it in the parking lot at work when someone is leaving. The unmistakable whine of a power steering pump can make even the most confident mechanic wonder when that car will breakdown.
Power steering pump noise is a unique noise that your car may make and once you've heard it you always know what it is. The noise is a mix of a whirring and a whining and will always vary with the speed of your engine whether your car is driving or in park. This whining noise can be subtle or extremely loud and may be worse in colder weather, or right when you first start your car.
Power steering pumps are most often vain style centrifugal pumps that are tasked with pressurizing the power steering fluid and pushing it down to your power steering gear or rack. As you turn your steering wheel it is this fluid pressure that aids in turning the wheels of your car.
There are a few reasons why you are hearing power steering pump noise from your car. First, it may simply be due to the design of the pump. Sometimes, noisy power steering pumps can continue to work flawlessly for years after they start making noise. This may simply be due to the manufacture's design or a tight tolerance in that particular pump. In this case, you would hear the noise but not see any other adverse effect like a loss in power steering, leaking fluid or a wobbly pulley. In this case, you may be able to help reduce your power steering pump noise by choosing a different type of power steering fluid, like a synthetic fluid.
Your power steering pump may also be making noises due to air trapped in the system. If the pump impeller is trying to move power steering fluid and starts catching air bubbles it can cause whining noises and vibrations to occur. Even tiny air bubbles you cannot see that are trapping in the power steering fluid may be causing the whining you are hearing. In this case, the best thing to do is flush your power steering system to try and remove the air from the system. Having air trapped in your power steering system can be difficult to diagnose so you may need to simply try a power steering flush to see if it solves your problem.
Power Steering pump noise may also be due to a failing power steering pump. As the bearings on the impeller go bad it will start to make noise as the pump is turning. If you have a bad power steering pump you usually will also experience leaks from around the pulley, a wobbly pulley, or difficulty turning the steering wheel when you are stopped. If you have multiple of these symptoms together, it is probably time to replace your power steering pump.
Lastly, and most usually, a whining power steering pump is an indication of a low fluid level. Your power steering fluid is probably something you have rarely if ever checked, so it often comes as a surprise if the level is low. Your power steering fluid reservoir is also relatively small so even a slow leak will cause a low fluid level relatively quickly. To confirm this is your problem it is as simple as checking your power steering fluid level. Some power steering reservoirs are clear so you can see through them to check how full they are. You may have to wipe away some dirt and grime to find the "full" line, but it should be easy to check. If you have an opaque power steering fluid reservoir there should be a small dipstick under the cap that will help you measure the fluid level. If it is low, then you have a leak somewhere and topping off the fluid should quite down your power steering pump.
If you discover a power steering leak, rather than simply continuing to add fluid every time it gets low, you should fix the problem permanently so you don't have to continue to check your power steering fluid level. Replacing lines or seals can be a surprisingly difficult task due to the location of your power steering pump and steering gear or rack. The easiest way to seal your leak is to do it from the inside out!

or check this link out for more info on this matter.
Power steering whining sqreeching Google Search

SIPHON OUT POWER STEERING FLUID.USING A HAND PUMP SIPHON TYPE YOU BUY AT ANY AUTO PARTS STORES.PLACE DRAIN PAN UNDER CAR NEAR POWER STEERING PUMP CATCH FLUID WHEN YOU REMOVE POWER STEERING LINES AT THE PUMP.REMOVE DRIVE BELT.THEN REMOVE POWER STEERING PUMP.YOU NEED TO TAKE OLD PUMP WITH YOU.TAKE NEW PUMP AND OLD PUMP TO MACHINE SHOP THEY WILL REMOVE POWER STEERING PUMP PULLEY FROM THE OLD PUMP TRANSFER PULLEY TO THE NEW POWER STEERING PUMP YOU NEED TO BUY NEW POWER.STEERING HOSES O RINGS TO GO ON NEW POWER STEERING PUMP.HOSES THE OLD ORINGS WILL LEAK. THEN TAKE NEW POWER STEERING PUMP ON INSTALL IN VECHICLE THEN BLEED POWER STEERING SYSTEM.FILL POWER STEERING PUMP UP NEW FLUID.WAIT FEW MINUTES.WHEN LEVEL DROP ADD MORE FLUID UNTIL YOU GET TO FULL COLD MARK.START ENGINE 15 SECONDS SHUT OFF CHECK FOR LEAKS. NOW CHECK POWER STEERING AGAIN ADD TO FULL COLD MARK.NOW RAISE FRONT WHEELS OFF GROUND.START ENGINES TURN STEERING LEFT TO RIGHT WITH OUT STOPPING IT DO SEVERAL TIMES THEN CHECK FLUID.ADD TO THE COLD MARK DONT OVER FILL.IF FLUID LOOKS TAN HAS AIR BUBBLES IN SYSTEM WAIT A WHILE UNTIL FLUID SETTLE CHECK FLUID LEVEL.DONT DRIVE VECHICLE UNTIL STEERING FEELS NORMAL.AND IF YOU HEAR NOISES.LOCATE PROBLEM FIX BEFORE DRIVING VECHICLE.

YOU NEED TO BLEED POWER STEERING SYSTEM BY HAVING LOW FLUID AIR ENTER THE SYSTEM.PARK CAR ON SOLID LEVEL GROUND SAFELY RAISE SUPPORT VECHICLE ON JACK STANDS.YOU CAN LEAVE CAR ON GROUND BUT TURNING STEERING WHEELS BACK AND FORTH WILL WEAR SPOT IN TIRES.TURN STEERING WHEEL TO THE LEFT WITH THE ENGINE OFF.CHECK POWER STEERING FLUID.ADD MORE FLUID IF LOW.MAKE SURE FLUID LEVEL ON COLD MARK ON DIP STICK.THE YOU CRANK CAR UP LET IDLE LITTLE HIGH FOR 15 SECONDS.THEN TURN OFF ENGINE.CHECK POWER STEERING FLUID LEVEL MAKE SURE ITS ON COLD MARK AT DIP STICK.THEN YOU START THE CAR UP WHILE IDLING TURN STEERING WHEEL IN BOTH DIRECTIONS SLOWLY TO THE STOPS DONT HOLD IN STOP TURN STEERING WHEEL LEFT AND RIGHT FOR SEVERAL TURNS.STOP ENGINE CHECK FLUID IF FLUID TAN STILL HAS AIR IN SYSTEM.LET CAR SET A LITTLE WHILE TRY THE BLEEDING PROCEDURE AGAIN.THERE IS A REASON POWER STEERING FLUID WAS LOW CHECK ALL POWER STEERING HOSES AT POWER STEERING PUMP TO THE RACK PINION ASSEMBLY.

FIRST OF ALL YOU HAVE TO REMOVE POWER STEERING FLUID.USING A SUCTION GUN OR LARGE SYRINGE.REMOVE MUCH FLUID AS POSSIBLE.WHEN DONE REMOVING POWER STEERING FLUID.YOU NEED A INLINE WRENCH AND OPEN BOX END WRENCH TO REMOVE THE POWER STEERING HOSE.YOU HAVE TO REMOVE POWER STEERING LINE FROM THE RACK.UNDER THE CAR SAFELY RAISE AND SUPPORT VECHICLE. THEN REMOVE THE TIRE OR WHEEL.THEN REMOVE THE INNER FENDER SPLASH SHIELD.ITS GOING TO BE A TIGHT FIT AND HARD PLACE TO GET YOUR HAND TO REMOVE THE POWER STEERING LINE FROM BOTTOM.WHEN DONE.MAKE SURE NEW POWER STEERING HOSE OR LINE HAS NEW O RINGS.WHEN INSTALLING THE THE NEW STEERING HOSE.WHEN ALL IS DONE BLEED POWER STEERING SYSTEM. FILL UP POWER STEERING PUMP TO CORRECT FLUID LEVEL.THEN LET IT SET FOR 2 MINUTES UNDISTURBED.THEN START THE ENGINE RUN IT FOR 2 TO 3 SECONDS.ONLY CHECK THE RESERVOIR AND ADD MORE FLUID AS NECESSARY.REPEAT THE OPERATION UNTIL THE FLUID LEVEL REMAIN CONSTANT.NOW RAISE FRONT OF THE CAR UNTIL THE WHEELS CLEAR THE GROUND START THE ENGINE AND INCREASE THE SPEED TO ABOUT 1500 RPM.NOW TURN THE STEERING WHEEL GENTLY FROM STOP TO STOP CHECK THE FLUID LEVEL.LOWER THE VECHICLE TO THE GROUND WITH ENGINE STILL RUNNING MOVE THE CAR FORWARD SUFFICIENTY TO OBTAIN FULL RIGHT LOCK FOLLOWED BY FULL LEFT LOCK BUT DONT HOLD THE SREERING WHEEL FIRMLY AGAINST THE STOPS.RECHECK THE FLUID LEVEL IF THE FLUID IN THE RESERVOIR IS EXTREMELY FOAMY ALLOW THE CAR TO STAND FOR A FEW MINUTES.WITH ENGINE OFF.WHEN JOB IS FINISH. WARNING DONT DRIVE CAR ON ROAD IF YOUR FLUID STILL FOAMY OR POWER STEERING PUMP MAKE NOISES.REBLEED THE SYSTEM AND CHECK FLUID LEVEL AND CHECK FOR LEAKS.DONT DRIVE CAR UNTIL YOU START FEELING NORMAL STEERING.IF YOU HAVE TROUBLE DONT DRIVE.HAVE IT TOWED TO THE NEAREST GARAGE.

some power steering system is computer control. if you disconnect battery the control module have to be program.the control module uses vechicle speed sensor and steering wheel rotation sensor.to operate an actuator in the power steering pump to control pump pressure and steering assistance.