Traffic lights went dark at busy intersections. Trains stopped in the middle of a workday. And long lines formed at A.T.M.s and grocery stores, where credit card readers stopped working.

Widespread power outages upended life for millions across Spain and Portugal (and briefly, parts of France) on Monday, leading to questions about what caused the disruptions and when the lights would return.

Here’s what we know.

Spain’s prime minister, Pedro Sánchez, said that officials were still investigating the causes of the outage. .

Portugal’s national energy supplier, E-Redes, cited an unspecified “problem in the European electricity grid.”

António Leitão Amaro, a senior official in the Portuguese government, said there was no evidence of a cyberattack. REN, a Portuguese electricity and gas supplier, denied reports that an unspecified “atmospheric phenomenon” was to blame.

There were widespread problems connecting to the internet and to phone networks across Spain and Portugal, and Mr. Sánchez urged people to make only brief calls on their cellphones.

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