@Vikki5
Have you tried sending a message to their mobile number ? I always make sure at the time of booking that the mobile number in their profile is the one used when travelling. You can also try and finding them on Facebook and use messenger, same with Whats App. It is also an idea to ask guests to download the airbnb app to their profile to receive push notification on messages.
If everything fails, contact airbnb to assist you. They should have an emergency number.
If everything fails, hang a note on the door with the contact number for the key holder and other instructions.