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

The hunt has begun for the cause of a massive blackout across Spain, Portugal and parts of France that halted trains, bank machines and traffic lights, in one of Europe's biggest ever power system collapses.

Electricity has returned to most of Spain and Portugal but with continued disruptions on the second day after an unknown failure caused a massive blackout across much of western Europe.