Happens to all writers now and then. You sit down to get some work done...and can't think of the next scene. Or paragraph. Or sentence. Or,dang it all, the next word.
Writer's block they call it. What to do, what to do?
Well, I think you have to let the tanks fill up, for one thing. But don't let the time go to waste. Read what you have so far. Tweak it here and there. When you get to the section before you were stricken, read it once. Then go back and read it faster -
You may launch yourself right back into the story.
However, don't confuse uncertainty with writer's block. Even if you've outlined your story (and I'm a compulsive outliner), you may not be sure that you're on the right track with your plot. If everything hangs up on you, your subconscious may be tugging on your elbow to suggest you may need to strike off in a new direction.
I often find that after a tough patch, one of the characters suddenly lights up for me. A minor player all of a sudden gets a break and becomes a star. Someone I'd been planning to kill off miraculously escapes. Or the opposite - someone I'm relying on to be in the last paragraph of the book suddenly drops dead and cannot be resuscitated.
At such times, go with the flow. Forget the outline. You may be on a better track now. And if you're not, if eventually you junk it and come back to the outline, you've got an interesting new character and maybe a good scene.
Worst comes to worst, sit there for an hour. Retype the last sentence you wrote. Do it again. And again. Fill the screen.
Finally, sheer boredom will break you out of the funk if nothing else will.