This is a community forum not monitored by Airbnb @Nathi0
You . need to call Airbnb straight away. All the ways of contacting them are outlined in the CONTACT AIRBNB post towards the top of the forum you are posting in.
How was the guest able to book if you don't have availability.
Airbnb takes host cancellations seriously and expect hosts to keep their calendars up to date to help avoid this. There are likely to be penalties, particularly if it is a last minute cancellation.