Some travelers were headed to weddings, some to funerals. Others were leaving for a Disneyland Paris vacation, taking a business trip to Britain, returning home from a honeymoon in Italy or moving to Australia.

Instead, on Friday, they found themselves sitting on airport floors, riding on buses or checking in to hotels as tens of thousands of passengers were stranded — unable to reach their destinations because of a power outage that shut down Heathrow Airport.

“It’s a crazy situation,” said Roxanna Bagherzadeh, who was missing Persian New Year’s celebrations with her family in London because her flight from New York never took off.

Some flights were rerouted midocean for Madrid or Montreal. Others were forced to turn back entirely. American school trippers were being held in London. Musicians worried they would miss their performances. Some travelers took circuitous detours across Europe to reach their families. Others had to give up on big plans.

“I was going to propose,” said Kevin Black, a bass player touring Europe with his band who was supposed to be joined by his girlfriend. But her flight turned around and took her back to Nashville.

“It was going to be on one of the bridges overlooking the river at sunset” in Paris, Mr. Black said of his planned proposal on their European trip. Now, he said, “it will probably be at a waterfall in Tennessee. What can I do?”

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