@Julie377 As you would know if you've read much of AirBnB's explanations of how things work (if you haven't, you should), when a host cancels a confirmed booking (for any but a very few "extenuating" circumstances), AirBnB blocks those dates to "punish" the host and discourage the host from cancelling in the future. They'll also fine you, with the charge to be withheld from a future payout.
Now, you made your mistake innocently and in good faith on behalf of your guest, so you should contact AirBnB immediately to explain. They will probably relent and unblock the dates this one time.
After you get this sorted, I strongly encourage you to spend at least a couple of hours reading through AirBnB's policies and procedures so you can avoid other costly mistakes in the future. Take notes. Seriously.
Best way to contact AirBnB is to start by sending a Direct Message (aka DM) to their Twitter account, @AirbnbHelp. Or, if you enjoy sitting on hold, you can try calling them: 415-800-5959 or 855-424-7262