ATLANTA — A sudden power outage caused by a fire in an underground electrical facility brought the world’s busiest airport to a standstill Sunday, grounding more than 1,000 flights in Atlanta just days before the start of the Christmas travel rush. More than 10 hours after the blackout began, authorities announced that electricity had been restored to several areas of Hartsfield-Jackson Atlanta International. Georgia Power said on its Twitter page late Sunday that power was back in the airport’s atrium and several concourses.