@Tracey104, what @Marzena4 is trying to tell you is that you may have received just a request for a booking (not a completed reservation).
Then you told the guest how to get into your listing.
The guest stayed for free (without paying anything to anyone).
When you asked, the guest just claimed to have paid.
If you go to the "Your Reservations" page, do you see this person as an Accepted Reservation?
If not, then you may have sent the information about your key to someone who never had a reservation.
It would be as if you walked into a hotel lobby and asked "How much are your rooms?", then the front desk person just handed you a key and walked away.