Cervical nerves. Paralysis of the diaphragm- the muscle to breathe gets it's innervation from the upper spinal cord through cervical nerves 2, 3, and 4. So fracturing the spine and injuring the spinal cord high above these nerves causes loss of respiration (like christopher reeves) and death. The instant death occurs in craniocervical dislocation- the spinal cord becomes torn at the junction of the brain.
Interesting quesiton. A broken neck may (it doesn't have to) crush the cord & instantly interfere w/ breathing & cause blood pressure to fall dangerously. Cause of death is asphyxiation and/or hypotension. The higher in the neck the injury is, the more likely damage to the cord will be fatal—though there is more room in the upper cervical spine where the cord sits & so injury to the cord is LESS likely higher up.