@Johanna246 if nobody cancelled the reservation then as far as Airbnb is concerned it's still on for the original dates. If you want to refund anyway you can send money via the resolution center:
https://www.airbnb.com/help/article/165/how-do-i-refund-my-guest
*Do not ever* cancel any reservation on a guest's behalf-- Airbnb punishes hosts for cancelling regardless of reason. In this case if the guest wanted to change dates and you chose not to accommodate that, it was the guest's responsibility to cancel.