Should an emergency or other unforeseen circumstance arise and you can't make it to the wedding after accepting the invitation, you should contact the bride or someone close to her (if you can't reach her) as soon as possible and apologetically notify the person that you'll not be attending. A polite cancellation will be greatly appreciated, as it'll allow your host and hostess to rearrange any seating, dinner arrangements, favors, etc., if needed. If it's appropriate, share the reason for your need to rescind the invitation, and make sure to send a nice wedding gift.