LONDON — It was supposed to be a new dawn, the moment when Jeremy Corbyn, Britain’s most left-wing political leader in a generation or more, proclaimed the start of the most ambitious economic transformation the nation had seen since the end of World War II.

Instead, a somber Mr. Corbyn was seen hugging his allies in the cafeteria of a sports and recreation facility in north London in the early hours of Friday, on his way to announce that his quest for power was over.

Like other left-wing parties in Western Europe, Labour is once again in crisis, scrambling for answers to seismic shifts in electoral politics. Labour’s embrace of the leftist vision of Mr. Corbyn — once seen by many members as the party’s salvation — has led to electoral catastrophe.

The question now is whether the party changes course and lurches to the center ground or elects a younger leader in Mr. Corbyn’s image, even after he was so thoroughly rejected by the voters on Thursday. Central to that conundrum is whether the voters rejected Mr. Corbyn himself, or his message.