Contact all people invited to let them know of the cancellation. Make sure that you have tried every type of contact (phone, text, email). Follow up until you actually speak to, or get a response from each person or party. This shows that you value the time and consideration you received from the folks you invited. MOST importantly, it gives you the opportunity to personally apologize for any problems the cancellation might cause them. If people are going to lose money (for example non-refundable airline tickets) it is your responsibility to make that situation right (yeah, you need to offer to pay).