The country’s top officials were trying to determine the cause of an outage that left tens of millions of people across the Iberian Peninsula without power.

A blackout brought much of Spain and Portugal to a standstill Monday, stopping trains, cutting phone service and shutting down traffic lights and ATMs for millions of people across the Iberian

Solar energy was powering the grid in Spain and Portugal on Monday—until it wasn’t, leaving tens of millions without electricity.