A coma is a state of unconsciousness where a person is unresponsive and cannot be woken. It can result from injury to the brain, such as a severe head injury or stroke.A coma can also be caused by severe alcohol poisoning or a brain infection (encephalitis) .People with diabetes could fall into a coma if their blood glucose levels suddenly became very low (hypoglycaemia) or very high (hyperglycaemia) .omeone who is in a coma is unconscious and has minimal brain activity. They're alive, but can't be woken up and show no signs of being aware. The person's eyes will be closed and they'll appear to be unresponsive to their environment.