@Gemma213
If your check-out time already passed you should go there and enter the apartment to see if your guest left with his luggage and see if he left the key inside your apartment.
If he left, took his luggage and your key with him then you should call him, text him and send him a message through Airbnb. If he doesn't respond then you should contact Airbnb and ask them if they can contact your guest.
If you don't get back your key today then you should change the lock and keys and then file a damage claim through the resolution center (send or receive money). Your guest should pay for it. If he ignores your claim or refuses to pay then you can involve Airbnb after 72 hours. I believe you will get paid.