Hi @Yasmin424 if your guest claims they didn't cancel their Airbnb booking but you're notified by Airbnb that it was canceled. You may advise them to reach out to Airbnb CS, there might be an ongoing issue in their account resulting in Airbnb canceling their booking. She can contact Airbnb CS via her Airbnb account or Airbnb app.
You may also contact Airbnb and ask the reason why it was canceled, but they will only discuss what you see as a Host in the reservation. Hope this helps!