Move to the side of the road as carefully as possible. It will require much more force to steer your car than normal.
Stop your car. Do not slam on the brakes! You may have to hit the brake pedal hard if you've got power brakes - the brake pedal will also feel different if the engine has stalled: stiffer and harder and lower.
Restart the vehicle and pull back onto the roadway if the problem has ceased. If the problem persists, use a cellular phone to call for help and wait inside the car or in another safe place.
Have your car checked out by a mechanic as soon as possible.