There is nothing automatic for a missed experience. Once the start time has passed, the operator has usually held a place, staff and equipment for you, and most do not refund a no-show.
Cancelling in advance is the only thing the site itself can do for you.
If something serious kept you away
Write to us, with the booking reference and what happened. We will put it to the operator. The answer is theirs, and we will tell you what it is rather than guessing at it in advance.
If you were late rather than absent
Contact the operator directly, using the details on your confirmation, as fast as you can. On a small group experience they can sometimes wait or move you; on a departure with a boat or a bus they usually cannot. Either way they are the only people who can decide it, and they cannot decide anything they have not been told.