In addition, the airline said that united.com is operational again and passengers can go on united.com to look at their flight status or re-book flights under its travel waiver for fees.
Flights were delayed and cancelled throughout the United States including San Francisco International Airport (SFO) where United keeps its hub.
United has been using twitter to give updates to passengers on the status of their systems.
Answering a question from twitter follower Adam WC regarding checking in for his flight on Saturday, United Airlines responded, Your flight tomorrow should be fine. Check the status on united.com and arrive early at the airport.