Even if you have a good reason, you would not get a refund if you missed your flight. Any ticket, even a refundable ticket, looses all value if you do not cancel prior to departure. Most airlines put a "Flat Tire" clause in their contact of carriage, which stipulates if you show up to the airport up to 2-3 hours late, they will still let you travel. You will however forfeit the value, meaning if you change your mind and don't travel, you won't even get a credit toward a future flight.
If you have a refundable ticket, and you cancel it in advance, you will be able to get a full refund. If you have any of the discounted tickets, and you cancel in advance, you can use the credit toward a future flight, minus a change fee, ranging from $50 to $250 depending on the airline.
I fly 150,000 miles per year.