Depending on the reasons as to why I cannot attend, my first option would be to have an associate cover me. This would be another photographer who would attend your day. They will be sent over all the relevant information, so they know what to expect and what you prefer. This means the day will run smoothly. if you have paid for a second photographer or have one included, they will still be attending. I will then edit your photos for you and all communication would still be with myself. Worst case if  I cannot commit for whatever reason,  ill refund you any money that has been paid or recommend another photographer who I can transfer the money to if you approve of them and they will take over your booking completely.