If a guest wants to cancel a booking you should never do it yourself, as you will have found you will have host penalties imposed including blocking out the dates, you have told Airbnb are no longer available by cancelling.
Do read up on host cancellations on Airbnb Help.
All you can do now is call Airbnb they will be able to see the guest said they wanted to cancel and you may be able to lift the host penalties.