Many of the traffic lights were turning from red to green again. The trains in Madrid’s subway system were rolling on all but one line. And baristas at cafes in the Spanish capital were serving café con leche to the few clients out on the quiet streets Tuesday morning.
Electricity returned to almost all of Spain by 6 a.m. local time, nearly 18 hours after the country and Portugal were hit by blackouts. The return of power left many relieved but also confused as questions about what had happened grew sharper.
A spokesperson for the electricity and gas supplier in Portugal, REN, said Tuesday that power had been restored to all substations of the country’s grid by late Monday and that everything was “100 percent operational.” All 6.4 million electricity users in Portugal were getting power, the Portuguese news agency Lusa reported.
The cause of the blackout, which stranded tens of millions of people on the Iberian Peninsula, remained unknown early Tuesday. Officials said there were no signs of foul play, such as a cyberattack.
Kristian Ruby, secretary general of Eurelectric, a trade body that represents the European electricity industry, said that it could take weeks or even months to complete the technical analysis required to fully understand the outage.
But some initial information has already emerged, he said. Around noon on Monday, a high-voltage connection line between France and Spain was interrupted. The power outage occurred just over 30 minutes later.
While that interruption would have been disruptive, it would not normally lead to a “system collapse” like that seen on Monday, Mr. Ruby said. Something more would typically need to happen, “like a sudden outage at a power plant, a sudden development on the demand side,” he said. “Then you can have an incident like this.”
European grids are struggling to cope with the increasing threats of climate change, Mr. Ruby said. But he warned against speculating too much about what had caused the problems in Spain and Portugal — one of the worst outages in recent memory, which he called “somewhere on the scale between a 50- to 100-year event.”
In cities across the two countries, life was returning to normal on Tuesday.
“It seems like everything is better today, but I don’t understand how something like this is possible with all the technology we have today,” said Doroteo García, an 87-year-old retiree, walking with difficulty near a Madrid train station. She had spent the day before trapped in her apartment, she added, because the elevator was not working.
“I lived off canned sardines all day because I couldn’t cook,” she said.
Though the power was back on, the Spanish capital was not quite back to its bustling self. Many people stayed home. Schools were open, but few with regular classes. Many parents who had rushed to pick up their children during the blackout on Monday did not return on Tuesday morning.
María del Carmen Sánchez, a school assistant at Cervantes Secondary School in the Lavapiés neighborhood of Madrid, said, “barely 5 percent of the students came” on Tuesday.
The blackout on Monday “was very confusing because we had no communication with the outside world,” she said. Some parents arrived very late to get their children. “Within the chaos of the situation, I think everything went quite well,” she said. “People were very patient, although there was some nerves and concern at first.”
In downtown Lisbon, parents dropped off their children at school on Tuesday morning, gyms had opened their doors again, and the sound of power drills filled the air.
In Murcia, a city in southeastern Spain, police officers who had been stationed at intersections to direct traffic were gone, and the traffic lights were working normally again.
María José Egea, 71, said she had gone to her regular Pilates class with a friend. She described the evening she spent in her seventh-floor apartment on Monday as “terrible.” But she said that her neighbors had come to check in on her.
“At first, we heard a lot of stories about the origin, and people were coming and telling me nonsense,” she said, standing out on a street in front of a busy coffee shop. “Everyone had a theory. The worst was the lack of communication.”
Azam Ahmed contributed reporting from Lisbon.
