Not quite clear by your post...what do you mean you gave her a full refund? How did you do this if you hadn't been paid for the booking?
Did you direct your guest to cancel through BNB. If so what would have happened is that BNB would have cancelled the booking in line with your cancellation policy. If you had a more strict policy they would have asked you if you wanted to refund the guest anyway.
You could then have said yes or no. Or you could have said you would offer a partial refund if you got a new booking to cover it.
In either case you would not have paid the guest back directly.
It sounds like you need to call BNB now to sort it out. Contact details are all outlined in the 1st post in this forum.