If you have tried through Airbnb messaging, their phone number and the Airbnb-generated email address, @MaryGrace2, and it is getting down to the wire, you can call Airbnb and have them try to get in touch. They will advise you on what to do if the guest cannot be reached.
Is the guest from overseas? It is possible they are travelling with no cell service or data. They may plan to find wifi the day before they arrive. That has happened here. It is worrying when there are keys to be exchanged, though. Did they give you a time of arrival when they booked?