If your vehicle is showing steering wheel is hard to turn in only one direction, here are the most likely causes and the parts you'll need to fix each one.
The steering rack uses an internal spool valve to direct hydraulic fluid to assist your steering. If an internal seal blows out on one side of the valve, you will completely lose power assist in that direction while the other direction remains perfectly fine.
The strut mount bearing allows the front suspension to pivot smoothly when you turn the wheel. If the bearing on one side of the car seizes due to rust or wear, it creates heavy physical resistance when turning the wheel in the direction that loads that bearing.
A severely worn, dry, or rusted ball joint or tie rod end on one side of the vehicle can bind up. This makes it physically difficult for the steering system to push the wheels in that specific direction.
On vehicles with Electric Power Steering (EPS), a failing torque sensor or uncalibrated steering angle sensor can confuse the computer. The system may 'think' you are only trying to turn one way, providing assist in one direction but fighting you in the other.
Hitting a curb or deep pothole can bend a tie rod or control arm. This severely alters the suspension geometry on one side of the car, making it much harder to turn the wheel in that direction due to extreme tire drag.