2009 Chevrolet Impala brakes problems

Stopped at the top of a snow covered hill. Put the transmission in 2nd gear and proceeded down the hill. The vehicle got up to about 15 MPH then lost all power; engine quit, no brakes, and the steering wheel locked in place. Because the brakes were not working the vehicle gained significant speed going downhill. Not being able to steer the vehicle because the steering wheel was locked it went off the road and hit a tree. Upon impact the air bags did not deploy.

I was deriving down a steep grade on the gatlinburg bypass toward pigeon forge, tn in second gear for engine breaking. I first noticed the car getting warmer and I checked the dual climate control by feel to make sure my rider had not increased the temperature. Then I noticed the steering was harder on slight curves; then I noticed the tachometer going to zero and sone other gauges decreasing; then I had to slow for a charper curve where I had no power brake assist and no powering steering assist.