How was it possible to book, @Gamini2? You should contact Airbnb support and explain it, but I doubt you will avoid responsibility. It is certainly not the guest's fault - tey couldn't have unblocked your listing.
I have heard that there is a bug on the site where blocked days become unblocked - I would suggest you contact Airbnb directly, inform them that this has happened and ask them to handle the cancellation, penalty free. Also, informing the guest to give them a heads up on the reason for cancellation and to explain that Airbnb will be handling it would be best.
A search on the CC will have many posts on how to contact Airbnb 🙂