What happens to the brain when someone gets knocked out?

Concussion: Loss of consciousness due to a blow to the head affects brainstem function, especially the reticular activating system, and shuts down awareness for a few minutes. Most concussions are self limiting, with effects totally gone within 2-3 days, but post-concussional pathology can last up to 6 weeks. A permanent brain injury may occur with or without loss of consciousness.