Almost all carriers require you to be at the departure gate by a specified time prior to departure. Your failure to do so will usually result in the cancellation of all remaining segments on the ticket. Whether there is any value to be refunded will depend on the fare rules for the ticket you purchased. For most fares, the refund will be 0.

To the suggestion that you might be able to pay a change fee, that generally only applies to changes made prior to the flight's departure. But, there certainly are full fare tickets which retain their value. They are just extraordinarily expensive.

Anything better is at the mercy of an agent.