This an unpleasant situation that can happen without a warning or a reason. So what to do?
I can imagine three common situation and they all have an advice in common. Don’t panic!
The first situation: you had a small crash, without human injuries. Don’t panic and don’t get too upset. You have insurance. Try to talk to the other drivers involved to see if they are ok. For this kind of accidents, in many countries the police presence is not required. Fill in the papers provided by the insurance company for this situation, take some pictures and exchange information such telephone number, email and name with the other drivers. Try to speak as calm as possible and without screaming. If you can’t get along with the other drivers, call or go to the police.
The second situation: you had a more serious crash but you are not badly injured. First of all don’t panic. Get out of the car and call the police and ambulance. With the help on the way, try to help the other ones but be very careful. You can do more harm by moving them so try to think first. Stop the fires, if you can. If no one saw that, try to get help but don’t get too far from the crash place.
The third situation: You saw an accident. First of all, pull on one side of the road and see how serious it is. If it was a bad car crash, call the police and ambulance, and try to help the others. If not, let the ones involved sort the things out. At most, you can write a statement about the accident as a witness.
As you can see, the best advice in to try not to panic. Keep calm and think the situation through. Ask for help as soon as possible and most important, don’t forget to be nice and respect the other drivers involved in the crash.