What causes a coma?

Brain damage: Coma is severe brain inhibition. Injuries to the brain, such as traumas, circulation compromise, inflammation, etc, can induce coma.

Coma : Think of the brain as having a wake and sleep center. When something bad happens to the wake center of the brain, it can no longer do its job to keep the brain awake in the spectrum of maintaining cousiousess. This can be from an anatomic/structural and/or physiological problem. Some of these are reversible and some are unfortunately not.