The coma

When someone is in the state of coma, it's similar to someone who's in a deep sleep, but you can't wake up this person just calling her/his name ou touching her/him. People who's in coma can listen, feel and smell, but can't answer you, the brain is functioning at it's lowest stage of alertness. Coma can be caused by many things, including: 

• a severe injury to the head that hurts the brain
• seizures
• infections involving the brain
• brain damage caused by a lack of oxygen for too long
• an overdose (taking too much) of medicine or other drugs

To help someone who's in coma to wake up, is treating him/her as a normal person, talking to him/her, visiting, etc. Even this person don't answer you (and that will happen).

http://kidshealth.org/kid/talk/qa/coma.html