The cut, which began just after 7:00 a.m., also caused short, localized losses of power in Paraguay, but didn't impact Tierra del Fuego in Argentina's extreme south because the region is not part of the national system, AFP reported.
The episode was the first time a power cut had affected the majority of Argentina, with a population of more than 44 million, and the entirety of Uruguay, which has 3.4 million inhabitants.
"These are failures that occur (even) with diligence. The amazing thing is the chain of events that took place to cause the total disconnection," Argentina's Energy Minister Gustavo Lopetegui told a press conference.
He said the outage took place "automatically to protect the system."
"We don't have any more information right now on how it occurred. We're not ruling out any possibility, but a cyberattack is not among the main alternatives being considered."
Hospitals, clinics on generators
Argentina's energy secretariat had earlier said the "interconnection system" had "collapsed," producing "a massive power cut" for which its generators had been unable to compensate, but that the causes had not been determined.
Sources from the official energy agency of Paraguay, which borders Argentina to the northeast, told AFP that cuts there had been "momentary."
A spokesperson for Rochester Gas and Electric Corporation (RGE), the biggest energy distributor in Brazil's southern Rio Grande do Sul state that borders both Argentina and Uruguay, said they'd had no reports of cuts.
Earlier in the day, power had returned to some sectors of Buenos Aires but the metro and trains were still halted.
Public hospitals and private clinics were running on generators.
"The only inconvenience is the elevators. We only have one working, but all services are operating without problems," said an employee at the Fernandez Hospital.
'This killed us'
Sunday was Father's Day in Argentina and some restaurants were expecting many customers.
"This killed us," Luciano Ferreira, the owner of a popular restaurant in the Boedo neighborhood of Buenos Aires, told AFP.
The restaurant had been fully booked and Ferreira had been expecting to make two or three times as much money as on a normal day.
Some people shared messages on WhatsApp with advice on how to prepare for a lengthy outage, such as collecting water.
In the interior plains region of Junin, residents stocked up on drinking water sold in supermarkets.
"Fortunately, we had two buckets on the patio that were filled with rain water. We've gone back to the Stone Age," Eduardo Gralatto told AFP.
In Cordoba, Argentina's second city, people headed to bakeries looking to buy supplies for the traditional Father's Day barbecue.
"We decided to open because we need to work, but early on we lost seven or eight tables of customers because we couldn't prepare coffee or bake bread," Carlos Arce, the owner of a bakery, told AFP.
Argentines also went to the polls in several provinces on Sunday to elect governors, with some local media reporting voters cast ballots by candlelight.
In Montevideo, some restaurants in the downtown area had power back by 11:00 a.m. (1400 GMT).
More than an hour after the blackout, UTE, the state power company, said its system was being brought back "from zero."
Argentina and Uruguay have a common power grid centered on the bi-national Salto Grande Dam, 450 kilometers (280 miles) north of Buenos Aires.