Many "non-refundable" tickets are usable for another flight on payment of a change fee, and the airline will issue a credit which expires in a year from the original ticketing date. It sounds as if you need to apply for a refund and submit the rejection, so that they can see if you have a credit.
United's refund
policy is here:
https://www.united.com/ual/en/us/customer/refund > click Refund Policies on the top bar.