Hello @Philippa25,
The article below describes what should happen if the host cancels when a guest has used a coupon.
https://www.airbnb.ie/help/article/1351
If the coupon code you have is not still active then you should contact Airbnb support.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.