Showing up late for a flight is not grounds for a fare change or refund. Most airlines don’t consider it a good enough reason. However, there is an unwritten rule that you won’t find in their conditions of carriage, known as the “Flat Tire Rule”, by which if a passenger turns up late due to reasons beyond their control, some airlines may schedule you onto the next available flight, either guaranteed to fly or as a stand‑by passenger.
The first thing to do is to find your airline’s stand at the airport. The airline representatives will let you know your options. They may allow you to exchange your ticket for a fee right there and then.
If the airline representatives can’t help, then you’ll have to contact the ticket seller. If the airline tells you to contact the seller, then contact them and find out what options the airline offers for your fare.
Important: if you missed the first flight segment and had a return ticket or your route includes a layover, then your following flight segments will be cancelled. Sometimes it’s possible to “save” them shortly after the flight departs. The ticket seller will also help you with that.