The blackout struck 7:07am local time (10:07 GMT), triggered by what Argentine government officials called a failure in a coastal grid that serves a number of countries, and a "total disconnection".
It paralysed transportation systems, closed shops, caused long queues at petrol stations, and delayed provincial elections in Argentina, forcing voters to cast ballots by flashlight or by the light of their mobile phones.
The lights went out as Blanca Brito was leaving her house to head to work at a hair salon in the capital, Buenos Aires.
"At that hour, you couldn't see anything. The bus was travelling in slow motion because the driver was afraid he might hit someone. It was terrible," said Brito, a manicurist, standing in the dark in the Crema Rusa salon as she waited for clients to arrive.
Aida Suarez, her colleague who recently arrived from Venezuela, said her thoughts went to her home country. "They are things that take you back, to live a little of what you have already lived," she said.
At the Parilla de Rolo restaurant in the capital, there was no power for the entire morning. But the lights came on around 1:30pm (16:30 GMT) and within an hour, hungry clients were streaming in amid the waft of meat on the grill.
Running the cash register, Adriana Rasgido sighed in relief. They had stocked up for Father's Day, and bills had to be paid.
"Five years ago, there were power outages all the time. I went five days without power at home at one point. But never at work. This is the first time I've seen anything like this," she said.
By midafternoon, nearly half of Argentina, with a population of 44 million people, was still without power.
"This is an extraordinary event that should never have happened," Argentine Energy Secretary Gustavo Lopetegui told reporters at a press conference in Buenos Aires on Sunday afternoon. "It's very serious. We can't leave the country without power from one moment to another."
Argentine power distributor Edesur said that the failure originated at an electricity transmission point between the Yacyreta and Salto Grande power stations, in the northeast part of Argentina.
'Nothing ruled out'
The government is not ruling anything out, including a cyberattack, although Lopetegui said it is not among the primary potential causes being considered.
Carlos Garcia Pereira, head of Transener, Argentina's largest power-transmission operator, said the failure in the system could be caused by something as simple as humidity during a day of heavy rainfall.
Lopetegui stressed that Argentina's power system is "very robust" and is generating more than it requires.
An investigation is under way to determine who is responsible, and if sanctions are necessary.
"It is important to clarify that this total disconnection happens automatically. It's the computers that run the system that do it, when they detect imbalances that could cause major harm, and in milliseconds the system disconnects in order to protect itself," said Lopetegui.
"There was no alert here," he added. "There was no possibility for an alert here because it's something that a human can't detect. There is no human intervention."
By 3:30pm (18:30 GMT) on Sunday, 56 percent of Argentina had its power restored, and most of Uruguay, with a population of 3.5 million, was back online. Uruguay officials blamed the blackout on "flaws" in Argentina's system.
Every province in Argentina, except for the southern-most Tierra del Fuego, which is on its own system, was affected.
In Neuquen city in west Argentina, most businesses were closed and street lights were out on Sunday. But at the Alto Comahue mall, one of the few places that were open amid the blackout, the generators rumbled loudly, blowing warm air into the rainy, winter morning.
The mall was a popular location for a Father's Day outing, and families camped out on benches and crowded the food court.
At a coffee shop, three young men hunched over laptops were making use of the internet.
"We work for a digital platform," said Nicolas Doguoli, adding that they don't usually work at the mall.