I booked a round trip flight through United from LGA to Denver. I could not make the initial flight on 8/20/15 from LGA so called to cancel that flight only. I took another flight on 8/21/15. Without my knowledge and consent United canceled my return flight for 8/23 without a refund. I showed up at the airport and they said I did not have a ticket. I was forced to purchase a ticket through delta for $535. United is telling me I cannot get reimbursed now.