Sarah,
Another reason to never, ever, ever cancel a booking. You may also be charged a cancellation fee.
I hope you have a message trail showing that it was the guest who wanted to cancel, not you. try 1 415 800 5959. But you will need to be patient and persistent.
If you use twitter I am told that is a very good way to reach Airbnb:
Step 1: Log into Twitter
Step 2: Search for @Airbnbhelp and choose "Follow" them
Step 3: Send a Direct Message (DM) explaining your issue and wait for their response. They may ask for information such as your listing but always provide it over DM so it is not public.