Damaged items by guests

We hosted 5 guests about a week ago and as part of our policy we send our guest a 'how the stay was' and goodbye & safe journey' message.

The response from these guests was 'your place was nice and we had a stay but the kettle is not working properly'. Guys, I used the same kettle for coffee just few moments before the guests checked in...one of them had either by accident when drunk or under some other influence put the electric kettle with plastic base on the electric cooker and melted the entire base that ALL the wires were clearly and physically exposed, which could have set the kitchen on fire.

I reported this to the guests and they said and sent me a screenshot of payment to us in London from Spain...up till now nothing is in our account. Told Booking.com for over 5 days now and nothing from them either