I am an EE not an MD, so I can't give the exact cause for a coma (I'm not sure if the exact cause is known). Basically there is damage to the brain (drugs, strokes, traumatic brain injuries, hypoxia, medically induced, etc). There are different levels of a coma (some are more profound than others). The severity of the coma and the length of time in the coma affect the likelihood of a recovery. Unsurprisingly, a lower severity and a shorter time spent in a coma result in a better prognosis.