There are days when the world seems to be rushing by or...pushing itself into our writing time. On those days, lack of time or excessive stress can make getting even one word on the page a struggle. But, I find that if I manage to type just one sentence, then I usually just can't leave that one sentence to stand alone. Maybe it's my tendency to think too much or my tendency to rattle on, but one sentence always becomes at least a few. And a few sentences can lead to more. It can at least leave me in a good place to begin the next day.
So, on the bad days when life intrudes into your writing time, strive to write one sentence. If that's all that will come, let it be. It's still one sentence farther into the book than you were the day before. Be forgiving of yourself if there's a lot happening in your life. Sometimes you just have to let life happen.
After all, where do all of our best ideas come from? They come from living.