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 dull and stupid and uninspired. Start writing sentence after sentence, describing what should be happening even if you only have an inkling. Or write what the character should be doing, thinking, or feeling. If nothing else, start writing about your blank brain.
Amazingly, the very act of putting pen to paper or fingers to keyboard sparks something. Indeed, it's almost miraculous. You'll fill the page with words, and, lo and behold, many of them begin to resemble what you should be writing. Sure, it's probably yukky, stinky writing, but it's still writing.
Your brain in an act of desperation will output words. It's like turning on a water faucet. At first the spigot is hard to turn, and the water trickles out. Some of the water might look rusty and undrinkable, but if you keep turning that spigot, the flow will increase and the stream of water will become sparkling clean.
Bad Writing Is Still Writing
You know what you do with that page of bad writing? You fill the page up, and you'll probably keep going by filling another page. Then you put those pages in the stack as today's work. You see, you can fix a page of bad writing, even really bad writing, but you can't ever fix a blank page.
Nora Roberts is often quoted as saying something along those lines. I doubt if she was the first author to state that insight, and I won't be the last. It's a universal truth in the writing world. Rough draft can be edited into finished draft – no matter how awful and pedestrian the rough copy may be. Blank pages can never be edited into anything – they'll always stay blank.
Learn how to refine gold from dross. Give yourself permission to write crap and then edit that into readable prose.