How was the guest able to contact you via What's App if they hadn't made a booking @Caroline1552 ??
Please have a look at Airbnb Help under host cancellations, you will be penalised for cancelling a guests booking unless they break your house rules. Airbnb Help is also good for all the basics around running your Airbnb you so have a look
Give Airbnb customer services a call and tell them that your guest is contacting you outside of Airbnb and says their account is locked. Can they confirm whether it is a legitimate booking.