a guest wants to reinstate a cancelled reservation

I had a booking from the 8th of July to the 15th. they arrived and said they were leaving as they thought they were renting the whole villa, therefore they want

I forgot to close reservation for few days when the house is unavailable and now I have a reservation I cannot cancel. How can I cancel it?

I am new to booking.com and i cannot hosta guest now aND hAve to cangel a reservation