Unfortunately BNB introduced a new way for guests to cancel which is confusing for hosts. The guests indicate to BNB that hosts have asked for the booking to be cancelled. They then send you a request to say that the guest have said you want to cancel. If you then agree rather than decline, it shows you have cancelled the booking.
Contact BNB now, hopefully on the messaging system it will show your guest wants to cancel. If this is the case you can demonstrate you didn't cancel and the review showing you cancelled will be lifted. (if you don't know how to contact BNB look at the first post under Community Help)
Good luck