We all have those time when the unexpected comes up and we didn't properly plan ahead. Often time we feel like we blew it. However, that isn't the case. One meal will not destroy everything you have been working hard for. Go into the next meal the way you normally would.
By taking it one meal at a time you don't allow yourself to "blow it" for the rest of the day or week. It is easier to recover from one bad meal then it is to recover from a weeks worth.