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When you sit down to write and words refuse to descend from your brain to your hand, that's writer's block. You can't picture the scene: it sits frozen on the screen inside your forehead or, worse, fades to static. You can't figure what would happen next. You can type "badger, badger, badger" for pages and your Muse remains asleep.
Causes of writer's block include sheer exhaustion, dehydration, but normally are a function of the Intenal Editor waking up and throwing itself between you and your Muse. You worry what you write isn't good, doesn't serve your purposes, isn't all it could be.
Of course, this being NaNoWriMo, you may simply be out of ideas. You have painted yourself into a corner and the plot or characters have sagged lifelessly. They don't seem to have any purpose any more.