Some days, no matter how disciplined you are, there will be interruptions and curve balls. You just cannot get started.
You have basically three options when that happens:
1. Get upset and try to bulldoze your way through the maze,
2. Write off the day
3. Keep calm and weather the storm.
By taking the third option you will find things to settle down toward the end of the day and you may be able to bounce back and do your best work toward the end of day.
Just like in sport, it is not over till it is over. In real life, the day is not over till it is over.