You stare at a blank sheet of paper with no clue of what you are going to say. Or, if you want to say something, you have no idea how to say it. Like any normal human being, you want what you write to be fairly good. And like any normal human being, you either write nothing at all, doodle, or write something that is horrible in your own eyes.
Everyone's done it. I still do it every day. So, whenever I find myself doodling hearts and lightning bolts and smileys in the margins of a sheet of paper, I do one of two things. I look out the window, or I start writing as fast as I possibly can.
The first option is better if you have no idea what you want to say. There's always something interesting out the window, even if you've memorized the view (or lack thereof). Is there wind? Are there trees? Then you have a plethora of things to write about. How's the light hitting the dirt and hills and fence? What color is the sky, precisely? Be as meticulous as possible. If you're telling the microscopic truth, as Brenda Ueland would put it, then what you write will be good.
The second option is what I do when I know exactly what needs to happen next in a story and it's just not working. Don't let your hand stop moving. Anything even vaguely related to the story will work. What's the weather like? What's the expression on your character's face when he notices the landslide? What does he say to the kestrel that's pecking at his head? What's it like to fly and fall and land on top of someone? Usually, I end up with a massive run-on sentence. That's okay. This is creation, not criticism. You can't edit when you write like that. It'll slow you down. There's a freedom that comes with putting down nothing but the original thoughts, the raw material of the story.
When all else fails, ask yourself these three questions: What story are you telling? How far can you see? And if you're not telling the truth, then what's the point? (I have these questions written down and propped up in front of me at my desk. They're lifesavers.) With 26 letters and infinite meaning, you can write anything.