If your reservation gets canceled or changed by the airlines more than twenty-four (24) hours before the first flight, call the airline program that originally booked your reservation. They will be the only ones who can further assist you, as no one else is able to make changes to the reservation.
If your reservation gets canceled or changed by the airlines within twenty-four (24) hours of the first flight, call the airline operating your first flight. At this point, that first flight airline has control over the reservation will be able to assist you However, if you're already at the airport, ask the airline staff for assistance, as they have more power to assist you than a call center agent.