This is a medical emergency. The person can die from heart or respiratory failure. Do not try to re-warm the person with direct heat like that from an electric blanket, fireplace or hot bath. This will direct blood flow away from vital organs and create another life threatening problem. Re-warming treatment must only be carried out by trained medical personnel.
Prevention is especially important with something as potentially serious as hypothermia.
Have a friend, neighbor or relative check daily, or see if your community has a check-in service for the homebound person. Contact your state or local energy office for more information. A person can reduce heating bills considerably by closing off heat to unused rooms in the winter.