In case your host is no longer available for a confirmed booking, the first option will always be to reschedule. Your host will offer you the opportunity to choose another time or date in their calendar directly, so that you can still do the experience together!
If rescheduling is not an option, your original host will cancel. If another host is available for the same experience at the booking time and date, you will be assigned a new host for the experience! That means that you will receive the same experience but from a different host.
In the unfortunate case your experience is cancelled and no other host is available, you will receive a 100% refund for the amount paid. You can learn more about refunds here.