You can go to the cancellation page and when asked why you want to cancel, pick the choice that says 'my host wants to cancel.' If your host accepts that, you will receive all your money back. Under no circumstances cancel as yourself or Airbnb will keep their fees.
If the host does not accept the cancellation, therefor agreeing that they are the ones who want to cancel, then contact Airbnb directly.
A final option is to dispute the charge on your credit card. Your bank will charge back to Airbnb and argue it out with them. You should prevail since the host has declined to provide the services that you paid for.