It's the one convenient excuse we all lean upon at one time. It is also a myth or, worse, a self-delusion. Writing and other forms of artistic talent are not magical gifts revealing themselves only when they want to take over our minds. Yes, writing is a talent, but so is the ability to analyze complex mathematics in one's mind. We never hear about physicist's block, however.
Professional writers write.
If you are an aspiring writer, then you have to adopt this discipline, too. Shakespeare wrote a lot. He did so because his audiences, and patrons, demanded new material. If he hadn't written steadily he might have found himself guiding a wooden plow behind an ox. At best, William would have been a merchant like his father.