This question always comes up. What to do?? My opinion is this....LIFE HAPPENS, allergies happen, sick babies happen, late meetings at work happen, the snooze button happens. So what. Keep in mind that you are on a journey to change your lifestyle not a 21, 30, 60 or 90 day diet and then go back to the old you. It's about the choices we make every day and the food you eat every day, and the improvements in your fitness you do every day. Does it matter if you skip one day, NO! Of course, rethink what is it that is getting on the way, is it a real reason, or is it an excuse, or just not finding the time to do it?
I have a few rules that I created for myself and they helped me lose my excess weight and help me today to stay on track and focused.
1. Never miss a Monday Workout.
2. Never go 2 days in a row without any type of exercise.
3. Workout at least 4 days of the week.
If you miss a day (or two, or three), just get back at it as if nothing has happened, starting with the workout you skipped. Miss much longer and you've got a decision to make. You can either start the program over from scratch, or ramp yourself back to the place you were when you stopped.
Ultimately, when you get back to your program, what you do is really about what will keep you motivated. That said, doing the full program as scheduled is the best way to make sure you get full benefit from it. If you just miss a few days, it's fine to resume where you left off, but if it's been weeks, you've probably lost most of your gains fitness-wise, so it's worth starting from scratch in order to get the most possible gains and to avoid injury.
And keep this in mind if you've been off more than 5 or 6 days. Start back up slowly or you'll risk getting so sore you can hardly move by exhausting all of your fast-twitch muscle fibers. Most programs offers a recovery week, a good idea would be to start with that.