A power cut halted hundreds of trains, forced hospitals and airports to use back-up generators and left 370 million people without electricity in northern India.
It hit a swathe of the country on Monday morning at 2100 GMT affecting people in Punjab, Haryana, Uttar Pradesh, Himachal Pradesh and Rajasthan states.
Most affected areas had power back by late morning, less than nine hours after the outage started. By evening, 15 hours after the outage began, officials said full power had been restored.
The blackout, one of the worst to hit India in a decade, highlighted the nation's inability to feed a growing hunger for energy as it strives to become a regional economic power.
It was suspected the country's northern grid crashed because it could no longer keep up with the huge demand for power in the hot summer, officials in the state of Uttar Pradesh said, but further investigation would take place.
The grid was drawing power from neighbouring grids as well as getting hydroelectric power from the mountain kingdom of Bhutan.
New Delhi's renowned Metro transit system, with 1.8 million daily riders, was forced to shut down for hours during the morning commute.
Some trains across the northern region were stranded when their electric engines failed. Others were delayed by hours as they were hooked to diesel engines.
Officials said restoring services to hospitals and transport systems were a priority.
The Confederation of Indian Industry said the outage was a reminder of the urgent need for the government to fix the power sector, ensure a steady supply of coal for power plants and reform the electricity utilities.
Transmission and distribution losses in some states are as much as 50 per cent because of theft and corruption by employees in the power industry.
India's Central Electricity Authority reported power deficits of about 8 per cent in recent months.
Power Minister Sushil Kumar Shinde deflected criticism, pointing out that the United States and Brazil also had huge power failures in recent years.
"I ask you to look at the power situation in other countries as well,'' he said.
But as many as one-third of India's households do not even have electricity to power a light bulb, according to last year's census.
Avnish Awasthi, Uttar Pradesh Power Corporation chief, blamed the grid collapse on states drawing more than their allotted power to meet the summer demand.
Shinde said he was not sure exactly what caused the collapse and had formed a committee to investigate.
Earlier this month, angry crowds blocked traffic and clashed with police after blackouts in the Delhi suburb of Gurgaon that houses many high-rise apartment blocks and offices. With no power in some neighborhoods for more than 24 hours, people erected blockades that paralysed traffic for several hours.