Puppies are lively, easily distracted, and often fail to answer their master´s commands. They are all the more liable to get run over by a car.
Confronted by this kind of accident, you need to keep calm and try to assess the observable injuries. Your puppy's pain and shock may make them behave abnormally, be cautious when assessing their injuries.
If your dog is bleeding profusely: press the wound with a finger, or, better still, with a clean cloth, to encourage coagulation. If your dog cannot get up, handle it with great care, trying to keep it horizontal. Spinal injuries may have been sustained, too much manipulation might aggravate this type of injury. If any bone is visible, do not touch it but cover with a cloth to stop any more germs getting in. Finally, regarding injuries to the muzzle, if there are blood or saliva clots interfering with breathing, remove them with a towel, taking care not to put your fingers between its teeth: it might unintentionally bite you under the effect of pain and shock.
If you dog appears unharmed after a traumatic accident: you must still consult your vet. Serious lesions can be invisible.
These recommendations obviously apply to any trauma : falling from a window, fights, etc.