Airbnb says to restore a cancelled booking you need to go back to the (cancelled guest) and offer them the dates again under a ' special offer'. Trouble is, because we have (mistakenly ) cancelled that booking, air bnb have blocked out the dates and I am unable to offer them to her ....
Judging by this help topic, @Deborah1067, it looks like the guest has to send you a message first, after which you can reply with a special offer. Guest can go into their cancelled reservations and choose the one at issue: