Yesterday we were planning on attending an engagement party for a friend of ours but at the last minute we couldn't make it.
The party was just over an hour away from us and we were told we could bring the baby but about an hour before we were about to leave for the island Dave got stuck at work with an emergency and we had to cancel. We felt horrible because we had RSVP'd yes and the party was formatted like a wedding - cocktail hour, formal meal.
If this situation happens to you and you are the hosts (bride and groom) be gracious and understanding. Don't start a fight with the guests - I'm sure they feel terrible that they can't make it. They may not go into detail but they probably have a good reason for not being able to come so don't quiz them, just accept what they say and say you're sorry you missed them and that you would have liked them to celebrate with you. If there is another pre-wedding party coming up mention that you look forward to seeing them at that event.
If you are a guest in this situation try and call the hosts (bride and groom) as soon as you know that you won't be able to make the event. If the decision to not go is made after the event has started, don't bother the hosts (bride and groom) but call a friend or relative who is there and let them know. They can tell the couple and you should email or text the couple stating that you won't make it and that you'll ring them after the event. If the event you can't make is the wedding just wait till the next day if you can't reach them before the invitation time - you don't want to ring them while they're about to walk down the aisle!
What did we do? Dave texted the groom that he was held up at work just before the party started and called a friend going to let him know what was going on. Then about three hours after the party was over, Dave called the groom to apologize and explain what happened. Then I went on the couple's bridal registry and sent them a nice gift -- spending a little more than we had planned.