@Julie-Ann8
1. You will need to cancel under Airbnb's extenuating circumstances policy or you will face cancellation penalties (see airbnb help centre)
2. All the ways of contacting Airbnb Help are posted towards the top of the forum you are posting in, in a post called CONTACT AIRBNB. It should also be on your booking confirmation and is there on the Airbnb Help Centre is you search 'contact Airbnb'.
For future reference if you know there is a problem which means you can't accept guests, then you should block out dates on your calendar, rather than let a guest book and then have to find alternative arrangements.