Do you go blank when you're staring at a blank page or today's equivalent, the blank monitor screen?
Laughingly, it does in movies all the time, but sometimes it does in real life too. Then it's no laughing matter.
Sometimes even when we writers have notes or an outline or some kind of roadmap, we sit down at the computer, lift our hands to the keyboard, and nothing happens. We're blank. Our mind is as clean as a wiped slate.
What do you do then?
You write anyway. Just start writing even if it's something inane and dull. Forget sparkling dialogue, beautiful description, and captivating narrative.
Then write. Write sentence after sentence, describing what a character should be feeling or what should be happening or write what you had wanted that scene to be about. As a last resort, write about why you think your brain is blank.
Something Miraculous Happens
I've seen this time and again. You'll fill the page with what you think are just words that mean nothing, but somewhere along the way, the words will start to take form and be germane to the story in some way. Your amazing brain follows through when you give it a task.
Sure, the writing won't be "finished" write, but you know what you do with a page of bad writing, don't you? You save it or print it and put it in the stack of pages if you work that way. You can always fix a page of bad writing, even really bad writing, but you can't fix a blank page.
Nora Roberts is often quoted as saying that very same thing. I doubt if she was the first to state that insight.
I bet every working writer who has earned a living has said or thought the same thing because it's a universal truth about writing for publication.
Rough draft can be edited into finished draft. No matter how awful and pedestrian it may be. Blank pages cannot be edited into anything.
Get the words down. Give yourself permission to write dreck if that's all you can manage. Then refine gold from that dross.