The United States and Canada are setting up a joint taskforce to try to find out what caused the worst power cuts in North American history.
Power is coming back gradually in cities like Detroit
The cascading blackout affected 50 million people in the US cities of New York, Detroit and Cleveland, as well as the Canadian cities of Toronto and Ottawa.
The Americans and Canadians, whose electricity grids are inter-connected, are blaming each other for the power failures.
Electricity supplies are gradually being restored - but problems are likely to run into the weekend and even next week.
Late on Friday, power company officials announced that all power had been restored to New York City, after more than 28 hours without electricity.
However, the subway will not re-open until at least Saturday.
The BBC's Emma Simpson in Manhattan says thousands of sweaty and haggard-looking New Yorkers faced a difficult journey home on Friday.
Many had spent the previous night camping in their offices, on park benches and on the steps of the main railway stations.
Airlines have started flying again after hundreds of flights were cancelled. But delays have been reported, as airlines try to get through the back-log.
The blackout, which began on Thursday afternoon, spread through eight US states and eastern Canada in a matter of seconds.
US President Bush has called the electricity cut a "wake-up" call and said the "antiquated" power grid must be upgraded.
The taskforce into what went wrong will be jointly headed by US Energy Secretary Spencer Abraham, and the Canadian Minister of Natural Resources, Herb Dhaliwal.
Correspondents say it could take weeks to discover the cause of the blackout.
New York mayor Michael Bloomberg said the power cut began in Canada, but Ontario Premier Ernie Eves said it was triggered somewhere along Lake Erie in Ohio.
Mr Bloomberg said an event in Canada had put extreme demand on the New York power grid and the system collapsed.
By Friday afternoon, 75% of Toronto's power supply was back up, but the electricity company has warned it may have to impose rolling blackouts until the system can get back to normal.
The city's subway was not expected to be running until late Monday morning.
Many businesses in Toronto and Ottawa remained closed and a state of emergency was in force.
Electricity was restored to the Cleveland area, but then turned off again due to rolling blackouts.
The city's water supply was also shut down because of the power failure.
RECENT US BLACKOUTS
1996: 4m people hit by electricity outage across nine states
1977: lightning strike leaves New York without power for 25 hours
1965: power loss in north-east US and southern Canada hits 30m people
In Detroit, officials urged people to boil water before drinking or cooking with it, because of low water pressure.
But nearly one million people in the city were still without electricity and faced another hot summer night without air conditioning.
Despite the problems, President Bush, visiting California, praised his country's emergency response system, saying work since the 11 September 2001 attacks to upgrade procedures had paid off.
According to the New York mayor, crime was down in the city on Thursday night, compared to the average night.
The three biggest US car manufacturers remained closed on Friday, affecting tens of thousands of workers.
General Motors suspended work at 17 plants in three US states and Canada and hoped to get back to work on Monday. Ford closed all 23 of its facilities across the region, while Chrysler Group closed 14 of its plants.