it is possible for a patient in a years long coma to awake and return to nearly normal life. it is rare.
I have heard of cases where a person in a ‘coma’ needed surgery and was administered anesthesia. after the sugery the person awoke completely from the anesthesia. in most such cases but not all, this doesnt last. sometimes it does. why doctors dont try to exploit this fact as a treatment, I don’t know. I also don’t know how deep a coma these patients were in.