You have to contact Airbnb. I have had this happen (guests forgot check out day). I went in, expecting them gone, but their stuff was still in the house along with their phones. Airbnb tried to contact them using other means perhaps?, said I couldn't touch their stuff but could start cleaning any room which was clear of things (hard), offered to cancel upcoming reservation (present guests would be charged in lieu). I started in the bathroom, didn't want to cancel incoming guest, luckily present guests appeared, all was well. Anyway: What to do:
1. Contact Airbnb immediately.