A coma is a state of unconsciousness where the person does not respond to anything, a natural coma can last any amount of time from days to years. In Heroes, comas have been induced by natural circumstances, drugs, and abilities.
Peter sits by Charles's bed as Charles lies comatose.
Peter falls into a coma.
Peter dreams while in a coma.
Peter awakens from his comatose state.
Sylar lies in a medically induced coma, attached to an ability negation system.
Angela lays in a coma induced by her husband.
Matt and Daphne attempt to wake Angela from her coma, but Angela convinces Arthur to let her go.
Many evolved humans are induced with drugs, attached to ability negation systems, putting them in a coma, after being captured by the government and placed upon Flight 195.
Danko brings Mohinder to the Building 26 coma room, and places him with the other evolved humans. Later, Danko shows Noah a comatose Eric Doyle. After she escapes, Tracy frees Matt, Daphne, and Mohinder from their comas.
Rachel Mills frees Eric Doyle, Mary Krause, and Gordon Hovey from the coma ward in Building 26 by pulling out their nose plugs.
Hiro and Ando free all the evolved humans from the coma ward in Building 26, and replace them with Building 26 employees.
Rebel frees Damon Dawson and Nana Dawson from the coma ward in Building 26.
When Sylar asks Matt to take away his abilities, the only way that Matt can do so is to put him into a telepathically induced coma where he lives out the nightmare of being the only man on Earth.
Peter enters Sylar's nightmare coma to fall comatose himself. After the two break though their collective coma, they realize that they have spent five years worth of life in the span of half a day.
In chapter 10 of The Agent, Rachel Mills frees Eric Doyle, Mary Krause, and Gordon Hovey from the coma ward in Building 26.