Tens of thousands of Delta passengers around the world were stranded Monday after a power outage at its Atlanta headquarters caused a global computer failure that halted all flights. Check-in systems, airport screens and even the airline's website and smartphone apps were affected by the meltdown, which began at 2:38 a.m. ET and lasted six hours. The airline suspended departures, with airport agents writing out boarding passes by hand. "Our systems are down everywhere," it told customers on Twitter.