At least 50 million people lost power in parts of eight states and Ontario, Canada.
Software problems in Ohio triggered the chaos, as well as a tree branch that started the outage, which cascaded across a broad swath from Michigan to New England and Canada.
Commuters in New York City and elsewhere had to sleep on steps, hitchhike or walk home, as trains were rendered powerless and gas pumps stopped working.
Food spoiled as refrigerators and freezers thawed, and jugs of water sold out as supply plants lost their ability to supply consumers.
Those in charge of New York's power grid say improvements since then make it unlikely an outage that size will happen again.
In 2003, New York lost about 80 percent of its generation in six seconds.