Much of Puerto Rico was without electricity Thursday following a fire at a power plant that set off a broader outage across the island’s aging utility grid, leaving most of the island’s 3.5 million people without service.The vast majority of Puerto Ricans don’t have generators and they were forced to spend the night in darkness and without air conditioning in the tropical heat. They awoke to find most businesses and public offices closed, with the officials saying it could be 24 hours before power would be restored.