@Steve662 It happens, they may be without cell service and can't reply. Have you tried to text, call them or WhatsApp if they are from outside the US? Sometimes guests just don't show up, I only had that happen once on a 1 night booking and they never replied and we never heard a word from them again and they never cancelled either.
Also you really should change your cancellation policy to at least Moderate. If this guy cancels today you get pay for today only even if he booked a week. He has no incentive not too change his plans. Make it cost him to hold your calendar hostage with bookings that get cancelled at the last minute. It's stops a lot of last minute cancellations.