A good question, as I have wrestled with this problem in the past. The answer is pretty simple:
You keep your butt in the chair, and you write anyway. The prose you put down may be lifeless and terrible, but that's okay. It will carry you past that moment and into a place where the writing will flow again. And later, when you're revising, you can tackle that difficult spot and make it as good as the rest of your story.
But you'll never have anything to revise if you don't first write something down, anything. You'll be grateful for that foundation, no matter how flimsy it may be.
So my advice? Write past it. You'll get past it; trust me.