Flights were grounded nationwide at about 6 p.m. Sunday due to a computer issue. The problem left some passengers stuck on airplanes for two hours, and others waiting in airport terminals.
International flights, as well as flights to Hawaii and Alaska, were not impacted, according to the Federal Aviation Administration.
Click here to view FAA advisory.
In a statement, United spokesperson Maddie King said: "We have issued a ground stop for all domestic mainline flights due to an IT issue. We are working as quickly as possible to resolve this issue and get out customers to their final destinations. We apologize for the inconvenience to our customers."