2 lane road coming downhill in the rain. At the bottom of the turn the steering wheel stopped responding, I clearly saw the driver (I was the passenger) attempt to continue turning. When she realized it wasn't turning she slammed on the brakes, but downhill in the rain it didn't do too much. We entered the opposing lane at I'd estimate 35-40mph (~65kph) then hit a large plain white work van head on. Drivers of both vehicles were fine but shooken up, the air bags did deploy. I managed to get a cracked sternum from the air bag and a broken collarbone from the seatbelt.
No idea why the steering wheel suddenly stopped working though.