BUDAPEST, Hungary (AP) — An unprecedented blackout across Spain and Portugal upended the lives of millions across the Iberian Peninsula, a stark reminder of modern society’s reliance on electricity, and how quickly life can descend into chaos when the grid goes down.

Electricity is pulsing through Spain and Portugal again after a massive outage knocked out power in both countries on Monday. So too are questions.

Spanish Prime Minister Pedro Sánchez says “no hypothesis is being ruled out” as he vowed to investigate and uncover the cause behind the massive blackout that gripped the entire Iberian Peninsula on Monday.