Coma is a medical term. It is a state of deep unconsciousness. People who are in that state cannot be woken up. They also do not react to pain or light. Coma can result from various conditions. Some of these are intoxication, poisons or damages or diseases of the nervous system. It can also be the result of certain drugs. Doctors sometimes put patients that have very bad injuries into a coma. There are different levels of coma. Comas usually do not last more than several weeks, but those several weeks could be disastrous. Sometimes only basic bodily functions are left over. Intensive hospital care is often needed to recover some of these functions.
Some comas can last years, but these are usually not woken up from. The family of a coma victim (or whoever has the power of attorney) can sign papers so that they are taken off of life support and die.