Contact AA. Unless you prefer otherwise, they'll most likely just rebook you on UA* (united express) which is still operating that route. And in that case don't be too alerted if they just ask you to show up at the UA* counter with your current ticket--given that they pulled out completely from the market, they probably have a blanket agreement in place with the other carriers to accept AA tickets against a reservation no questions asked (although I would call UA to ensure that everything looks OK in their system, just in case the AA rep forgot to do something like manually entering the ticket number and that will cause the reservation to auto-cancel).