Your rooms on Booking.com can happen to be hard-closed (as below screenshot), and the process to re-open can be done via your Booking.com Extranet.
Make sure you are not currently connected to any Connectivity Provider (with Hostfully being one) during the process, and no worry, you can always reconnect afterwards.
Step 1: On Extranet, under Account > Connectivity Provider, you can Deactivate Hostfully as current XML connection.
Step 2: Under Rates & Availability dropdown, choose Open/Close Rooms and specify accordingly (make sure the date range is specified).
Step 3:. Under Rates & Availability dropdown, go to Calendar to make sure the dates are open now.
Step 4: Go back to Account > Connectivity Provider, and connect to Hostfully again [Instruction].
P/S: If you find any difficulties throughout the process, you can contact Booking.com LPS team via Extranet.