It's highly unlikely that a registered marriage celebrant would cancel at the last minute and in my humble opinion, quite unethical. It would have to be a major crisis for a celebrant to cancel. If a Celebrant cancels at the last minute, it is up to the Celebrant to find a replacement Celebrant for the couple.
All my couples are informed that I have a back-up Celebrant organised should there be any crisis that prevents me from turning up at the last minute. As I am always at the venue about 45 minutes prior to the ceremony, there should be plenty of time to notify my back-up Celebrant if, heaven forbid, I have some sort of car accident on the way for instance. She will have a copy of the ceremony and will be able to bring along the documents required.
In my car I have a notice that has all the relevant details about the ceremony I am heading to, including phone numbers for the venue, a contact number for at least one member of the wedding party and the number for my back-up celebrant. In the unlikely event of a car accident, I am relying on the emergency services person to make the calls for me. So, if I cannot get there, there will be a celebrant who will arrive as soon as physically possible. By the way, all couples need to have some form of photo ID on them (or in someone’s possession) so that the replacement Celebrant can verify who they are before proceeding with the ceremony.
Here's hoping it never happens, but a good celebrant is prepared for everything!