Peugeot 407 car slowing down and not starting

I got to a T- junction and a man suddenly jumped across me. In order to avoid hitting the man I suddenly braked the car. I can not remember clutching. Then the car jecked and stopped by itself. I tried starting the car and it did start noly for it to run a short distance, slowed down without accelerating again and stopped. When I start the car, it starts but does not stay for long and stops. We had to tow the car to our mechanic who used a computer to find out what the was and said therewas no indicating error. But he said its a mechanical fault. What can the problem be?