From CNN: [cnn.com]
(CNN)United Airlines grounded all domestic mainline flights Sunday night because of a computer problem, United spokeswoman Maddie King told CNN.
"We have issued a ground stop for all domestic mainline flights due to an IT issue," King said. "We are working as quickly as possible to resolve this issue and get our customers to their final destinations. We apologize for the inconvenience to our customers."
Mainline flights are those operated by an airline's main operating unit, not regional alliances or subsidiaries.