Alanna Gladstone, a 40-year-old film editor, had landed in Portugal’s capital, Lisbon, on a flight from New York hours before the outage. She checked into her Airbnb and took a nap; by the time she woke up, the technology that the country takes for granted had gone dark.

The sweeping power outage that hit Spain and Portugal this week has raised questions about the electricity grid in a region not normally known for blackouts.