If their vehicle has come to a stop on a roadway, the person should do everything they can to get their vehicle off the roadway as far as possible. If their flashers are operational, they should make sure they are on and, of course, the head lights and tail lights are on also.
If the crash was caused by someone else, it is always wise if the person can take photographs, perhaps with their cellphone. If anybody is injured, they should call 911, which is going to signal first responders, traffic police, and ambulances for people that are injured.