United Airlines has said six flights were cancelled and 200 more were delayed because of a computer problem which grounded all of its US domestic flights for more than two hours on Sunday.
Company spokeswoman Maddie King called the glitch an "IT issue".
The airline is waiving flight change fees until Wednesday for any travellers affected by the problem. United said there will be no extra charge for anyone flying in the same class and between the same cities.
Chicago-based United Airlines and United Express operate more than 4,500 flights a day to 339 airports across five continents.