@Lee107
A few options - they can ask AirBnB for a refund due to extenuating circumstances, in which case if AirBnB agree they will also get their booking fee back from AirBnB.
You can ask the guest to cancel - (however if you receive and email asking you if you are happy to cancel decline it) and you will be paid subject to your cancelleation policy. You can then refund them what you wish. In addition your diary will be open for bookings. They will not receive AirBnB fees back.
You can do nothing, you will get paid as if they arrived and stayed. You can refund as you wish - but your calendar will remain blocked as AirBnB will believe you have your guests staying with you. They will not receive AirBnB fees back.