2010 Chevrolet HHR drivetrain & axles problems

In a parking garage on the 4th floor. Backing out of space. While backing up, I hear a loud boom. Nothing behind me. Thought perhaps a tire failure. Put vehicle in drive to pull forward out of traffic lane, and the engine accelerated uncontrollably. Brakes wouldn't respond. By the grace of g-d or reflexes, I turned the wheel to the right to avoid flying through the guardrail and off the 4th story of the parking garage. Vehicle came to a halt when it smashed into a concrete support pillar.

Tl* the contact owns a 2010 chevrolet hhr. While driving approximately 20 MPH, the vehicle decelerated without warning and stalled. As a result, another vehicle crashed into the side of the contact's vehicle. A police report was filed and no injuries were reported. The vehicle was not diagnosed or repaired. The manufacturer was not notified. The approximate failure mileage was 55,999.