While this is rare, a speaker can still cancel just before the session is about to start. Even though as an organizer, you might feel like there's no good enough excuse for this, speakers are only human and emergencies happen. Below are a few actions that can help you to prevent such an occurrence:
Action 1: Book a backup speaker
If you have invited professional speakers to your event, leverage their industry network to find one or two more speakers. Discuss with them and see if they are willing or available to be backup speakers. Otherwise, talk to a few of your own speakers and see if they are willing to do an extra session just in case.
Action 2: Create an "Expert Panel"
After the event starts, find the most interesting people in the audience and do a round-robin about their ideas about important topics in their industry with a newly expert panel
Action 3: Create your own session
If all else fails, you can try to create your own session, where you are the speaker.
If all these do not work you can reschedule the event.