@Wendy700 You'll need to get in touch with Airbnb CS to deal with this, and you should let the guest know that you're very sorry, but there was a calendar glitch and the dates he booked are actually unavailable. If he's still within the window to cancel with a full refund, maybe he will be amenable to just cancelling himself, in which case you won't need to deal with customer service.
Other hosts have reported here that this same thing has happened to them. Sometimes the calendar appears blocked on the host's end, but available on the listing itself. Check all your settings and if it happens again, hopefully Airbnb can figure out why this happened.
