Can a major atv wreck(bleeding in brain, concussion, coma, etc.) stunt your growth?

Theoretically : Theoretically a severe brain injury affecting the hypothalamus and hormonal regulation could have an effect on growth and other hormone mediated processes.

Intracranial bleed: Depends on the age of the patient at the time of injury - if an adolescent then one is through most of the growth at that time, in a younger child still growing then yes a serious brain injury can slow one's growth.