@Susan1196Did you attempt to contact the guest at all to determine if they weren't coming, and if so, was it through the Airbnb messaging?
I had this exact scenario at Christmas. A family booked for 3 nights, then never showed. I tried every contact method to find out if they were still coming, with no response. I got quite worried, and called Airbnb and let them know that the guest was a no-show. The weather was quite bad, and I was concerned that something could have happened to them. Airbnb looked at the account and said that as they could see that I had made an honest attempt to contact the guest, if they asked for a refund, they would not receive one, as they had never reached out to let me know their plans had changed. Sure enough, they never showed up, and I kept the money.
If the guest is just asking you at this point, I would just tell them no, explaining that their booking blocked your calendar and you didn't have the opportunity to host another guest.