Friday's blackout just before 5pm stopped traffic lights from working, plunged Newcastle Airport into darkness, affected Ipswich Hospital and caused huge disruption on the railways, leaving thousands of passengers stranded.
There are also further delays affecting some trains this morning, with London North Eastern Railway having cancelled several services on its Twitter feed.
The National Grid Electricity System Operator said issues with two generators caused the loss of power, affecting London and the South East, the South West, the Midlands, the North West, the North East, Yorkshire, Cornwall and Wales.
Ofgem said on Friday night: "In any incident the priority is to get power restored to customers as quickly as possible. National Grid has now informed Ofgem that the system has been restored.
"However, Ofgem understands the frustration this power cut has caused consumers. Ofgem has asked for an urgent detailed report from National Grid so we can understand what went wrong and decide what further steps need to be taken. This could include enforcement a*ction."
Professor Tim Green, co-director of the Energy Futures Laboratory, Imperial College London, believes the two generators disconnected were at Little Barford and Hornsea.
He said: "The first generator to disconnect was a gas fired plant at Little Barford at 16.58. Two minutes later Hornsea Offshore wind farm seems to have disconnected.
"This might be linked to disturbance caused by first generator failing; might not. We will need to wait for National Grid's full technical investigation to get to bottom of that."
Around 300,000 UK Power Networks customers were affected in London and the South East and Western Power Distribution said around 500,000 people were affected in the Midlands, South West and Wales, with power restored to them all shortly after 6pm.
Northern Powergrid, which serves Yorkshire and the North East, said 110,000 of its customers lost power, while Electricity North West said at least 26,000 people were without power in the North West of England.
Ipswich hospital was hit, with a spokesman telling Sky News: "There was a national power outage and Ipswich Hospital was without power for about 30 minutes, all of our teams kept all of our patients safe, and we are now back up and running."
Newcastle Airport was plunged into darkness for about 15 minutes.
One passenger, Scott McKenzie, said: "It was a bit worrying to start - more so because various alarms were going off and staff clearly didn't quite know what was going on either.
"Some of the spaces in the airport have little daylight - we were literally plunged into darkness and people were using their phones as torches to see and get around."
There were chaotic scenes across England during Friday's peak rush-hour, with traffic lights down and trains coming to a standstill.
Frustrated passengers stranded at King's Cross in central London after the outage branded the lack of help as "astonishing".
Large crowds of families with children, elderly people and others with disabilities were among those left waiting hours for information after all services in and out were cancelled.
When the first northbound service left the station at around 9.30pm, passengers were filmed forcing their way through the barriers in an attempt to get themselves on to it.

In south London, customers described "apocalyptic" scenes as the power went out in Clapham Junction station.
Harriet Jackson said: "(I) realised that nothing was open and there was hardly any phone signal.
"All the traffic lights were down, but there were no police present, which meant it was dangerous to cross - cars weren't stopping either.
"It was like witnessing something out of an apocalyptic film.
"No one knew what was going on and, given it's a Friday afternoon, it's the last thing you want to encounter."
Other train services in and out of London, including Thameslink, Southern and Gatwick Express, faced delays and cancellations because of the disruption.
London North Eastern Railway (LNER) said a number of its trains had broken down between London and Stevenage, suspected to be because of the electrical supply problem.
It also told customers not to travel from Kings Cross and said any tickets for Friday would be valid on Saturday.
"Services heading towards London are now able to depart however due to congestion across the network it will take some time to clear the backlog," LNER tweeted.
The Metro in Newcastle was affected, with delays carrying on into the evening. Those travelling to the city's airport were advised to allow more time to make their journey.
Transport minister Chris Heaton-Harris tweeted: "This is an incredibly difficult and frustrating evening for rail passengers.
"I have spoken to @networkrail who have mobilised more staff and are working tirelessly to get people home safely and as quickly as possible.