@Roxanne69 I wouldn't worry about Airbnb at the moment: they're not going to come to your place and get rid of your guest and/or their belongings. I'm assuming you've already contacted your guest via the Airbnb messaging system? If not, I would message them and ask them about their checkout. I would also try calling them. I'm assuming they were not at the property when you went there.
How long was the guest staying for? If it was just a short stay, I would send them a message via Airbnb messaging saying that if you don't hear from them by x day/time, you will remove their property from the unit and store it. I would also secure the unit so they can't re-enter without you, if that is possible.
If the stay has been long enough that they could be considered a tenant in your area, things get more complicated.
However, the first step is contacing the guest.