Could also be the travel or endpoints on your radio, they could be dialed down too low for some reason. Settings on radios can get changed accidentally after all.

Another thing is it could be a weak servo. If it can turn the wheels while the car is held up in the air but not while it is sitting at rest you may need to get a new servo that puts out more torque.

A third thing is could be damaged servo, blowing the gears out can limit steering in one or both directions but the servo will sound ugly, not the hum you are used to but popping grinding sounds.