By late Friday evening, the computer systems started to come back and flights started to resume, but not after causing a huge delay.
According to a statement issued by the airliner, the airliner experienced a computer outage around 5:15 p.m. that triggered flight delays and issues with reservations.
"We apologize for the disruption being caused to travelers at affected airports and we are seeking to resume operations as quickly as possible," United Airlines said in a statement released Friday evening, adding that the airliner apologized to passengers for the disruption.
Some passengers on Twitter had reported flight delays or cancellations at San Francisco International Airport.
"I'm at SFO at the moment, trying to get home to Chicago," passenger Jason Huggins said via Twitter. "Flight was United #669, scheduled to leave at 6:57 p.m. I arrived at the airport around 5:40 p.m. Computers were already down by then. Officials came by to explain the scope of the problem."
Joseph Andreasen said he was stuck while waiting for a flight out of Denver's airport.
"No Continental or United flights leaving. System down nationwide," Andreasen said via Twitter. Continental flights are operated by United Airlines.
Marc Balcer was stuck in Portland, trying to get back to the Bay Area. He said an airline employee told him this is the longest computer outage they have experienced.
There's no word yet on what caused the outage.