CARACAS—The biggest blackout in Venezuela’s history entered its second day on Friday, as dialysis patients went without treatment, airlines canceled flights and factories and schools shut their doors in a nation reeling from an economic collapse.

Electricity went out Thursday afternoon, plunging some 30 million people into darkness in a country mired in political turmoil and hyperinflation. Power was restored only briefly to parts of the capital on Friday afternoon before going out again.

The...