It can't be booked without you knowing. If there is no reservation on your calendar, they have not booked. Depending on how nice you want to be you could just tell them to find their confirmation email and/or confirm the booking on their calendar and if it shows booked tell them to contact AirBnB at the number in their confirmation. If you wanted to be nice you could call AirBnB and have them call the guest and guide them through the process. However, I would warn them that until you you receive a confirmation from AirBnB, as far as you are concerned they do not have a reservation and will not have access to your property. Also if someone else books before they get it resolved the property will be unavailable.