Writer's block is a myth, dating back to the Lethargy Cults of ancient Athens. A writer by definition is one who writes, and if you find that for some reason you are unable to do so — well, the algebra isn't hard. The professional creator creates and creates every day, because that's the job. But there is such a thing as a slow day.
The slow days, the grey days, the stable and the mucking days. It's not like you don't produce something, ‘cause you do — it's just that you're a little less magic than usual. Usually you’re John Constantine, but you find yourself stuck being, say, Harry Dresden. Which still does the job, mind. And maybe that's the work you redo later. Some days all you can do is lay down some foundation while Lights Out plays noiselessly in the background.
These are, of course, the days that make those other days so high and holy.