@Jenny127,
Hi Jenny, golly gosh, the potholes that we sometimes fall into. We can hear that you are distressed, and somewhat surprised as this was not something that you seem to have expected. Sorry about that.
This is quite a common thing for guests to do, because they put keys in their pockets or wallets to keep them safe, and not loose them. If hosting, then it's only a matter of time before it happens with one or more guests.
Type in the word "keys" you will see how many discussions have already occured on this hot topic. You will see lots of ideas as well, and possible solutions for the future. [eg.go to > https://community.airbnb.com/t5/New-Hosts/took-my-key/m-p/193343#M18631 ]
If you can continue try to contact your guest via phone, skype whatsApp or Airbnb email, Guests may be better placed than Airbnb to help you sooner. If it is an accidental removal of the key, then your guests are not going to re-use it or pass it one. Send a message perhaps and ask them to dispose of it. It's not safe to post it! get spares cut if that's the type of key you use, or invest in one of the new automatic locks recommended by other hosts.
If your guest has gone home to another country, it is highly unlikely that your home security is at risk, perhaps this is some comfort.
best regards, Christine.