Updated 08/28/12 – 9:36 p.m.

CHICAGO (CBS) — United Airlines’ reservation computers are functioning again, and service is back to normal at O’Hare International Airport, following a three-hour, worldwide reservations computer outage that created chaos for passengers.

From 2 p.m. until 5 p.m. Tuesday, customer service representatives were on their own, writing tickets by hand as lines in Terminal 1 grew to lengths as bad as any at O’Hare on Thanksgiving Eve, traditionally its busiest day of the year. But the agents’ computers began to flicker to life shortly after 5 p.m., at kiosks about 5:15 p.m., and minutes later at United’s mobile site.

Despite that, the damage was done for some who counted on being able to print boarding passes and check luggage at O’Hare.

LISTEN: WBBM Newsradio’s Bob Roberts reports

“I’m probably not getting home tonight,” said David, who missed a flight to Bowling Green, Ky., after finishing a training course. “I’ve been here since Saturday and I’m definitely missing my family.”

Others were less philosophical. One woman argued with customer service representatives, demanding to know why United didn’t have a better back-up system.

A veteran United employee said he had seen spot computer outages before, but never one that affect the airline’s entire reservations system worldwide.

United has struggled with reservations computer problems on and off since March, when it combined its reservations systems with those it inherited in its merger with Continental Airlines. The airline said it would rebook any passenger who missed a flight at no charge.