Not clear if it was an official booking request or just an inquiry by message. if it was a booking request you had 24 hours to answer, either accept or decline. you may have innocently believed you were just waiting for the validation to come through, but the clock was ticking.... in that case, not answering may have caused the blocking of those dates. i would call Airbnb, explain the situation, and chances are they will unblock those dates and the original guest can book. good luck!