@Gemma42
Unfortunately it sounds like many new host, you have rushed in a little too quickly and not read up on how AirBNB works before letting your listing go live.
A host should only cancel a booking if there are extenuating circumstances. BNB takes host cancellatios seriously and penalises a host who does this including blocking out dates in your calendar - as you have indicated these dates are no longer available.
What you should have done is got the guest to call Airbnb and explain they had made an error in the booking and asked BNB to amalgamate it into two nights.
Hopefully BNB will be able to see through the messaging system what has happened. You can ask them if they will make an exception and lift the cancellation ban. However they may not.
Give them a call now. 2nd post Community Help forum for contacts.
In the meantime you will probably want to put your listing on hold (snooze it) while you read through the Help section on their website and the useful Community Guides in this forum.