x5 was crashed from behind?
 

my x5 was crashed from behind and i was told that bmw's have a sensor that when you get hit from behind the sensor shuts off the car. My car won't start now and I want to know it this is true? If it is how can I reset this?

Yes, there is an emergency disconnect that triggers (blows) on the battery terminal on impact. It reduces the chances of sparks causing fires. You should get the dealer to replace it. There may be more to it than just R&R - I suspect some electronics may have to be reset.