2001 Dodge Caravan Hard to turn left & knocking sounds

I drove around today, and all of a sudden the steering wheel locked and it was hard to turn the wheel left. I offed the car waited about 15 minutes and started the car. I drove home, able to turn right easily, but when turned the wheel to make a left it was difficult to steer the wheel, I also heard a knocking sound when making left turns. Please help me and respond A.S.A.P.
thank you very very much !