Try this one on. I was once in midstream, about a year and a half into writing a book, my first significant, major publisher book, when my editor left and I was reassigned to another editor. Met with him for lunch. I had 150,000 words in my hand, what turned out to be about half the first draft of what later ended up being a 250,000 word book.
Handed it to him. He glanced at it, dropped it on the floor and sniffed, "This is a picture book. I want maybe 20,000 words."
I walked out, stunned. Career done. A year and a half wasted. Started drinking. Kept drinking. Twelve hours later I e-mailed my former editor, and was surprisingly lucid. She called me the next morning, said to hang loose, she was making some calls. The publisher called me an hour later, apologized. Pulled the editor off the project.
The book eventually built my house. But it was one hell of a 24-hours, one I care never to repeat.
That being said, every time I finish a book, I feel like there are no words left in me, nothing with a shred of originality, the most innocuous phrase sounds contrived, the most un-innocuous sounds pretentious.
It takes 6-12 months for me to write long-form again, for the language to refresh. It's like my brain goes dry, and no blood flows through my fingers.