Steering suddenly went out

A few times in the past two days, when turning at slow speeds (like onto my street or into my driveway) the steering would suddenly lock to the point where I couldn't complete the turn. The first couple of times it came back after a few seconds or if I turned the wheel the opposite direction. But just now, turning in my driveway, it completely locked and I couldn't turn it AT ALL in either direction. No warning lights come on and the car is functioning perfectly otherwise. I turned the car off, waited a while and started it again and the steering wheel still won't turn at all. What is that about?

I have a 2003 classic with 57K miles. I bought it used last summer and the only other problem I have had with it was a shifter sensor that came loose and when they repaired that, they also said I had to replace the auxillary battery.