The outage lasted for two hours. While not operationally involved with aircraft safety, the outage impacted boarding passengers in some cases.
The airline says the problem was caused by what it calls a network outage. It is now trying to get operations back to normal, but there are still reports of long lines at some airports.
The United website was down but is now working again.
Earlier Tuesday the airline was holding flights destined for San Francisco, Houston and Newark.
United has had repeated problems since it merged its computer systems with Continental in March.