I don't really think any writer is immune to 'blocks' from time to time. In my book, The Way of Story: the craft & soul of writing, I devote a whole chapter to the subject: "Transcending Writers Block". In this chapter, I explore alternative perspectives on the subject, some unexpectedly positive.

          Writing, like life, is a process.   Best, Catherine Ann Jones www.wayofstory.com                  

Wow, Paula & Cate, you wrote more than I did! Thank you for your wonderfully personal and pertinent contributions to the topic. I can identify with both of your experiences completely. Well, maybe not being flat out watching Six Feet Under, but the method is a familiar one to me :)  I also can connect with Paula's letting her characters take her where they want to go. It's magic, sometimes, when that happens. Thank you both again - what a treat to see such lovely comments!

I was a professional writer (business and high tech) for 36 years, and in that job I would say, smugly, that I didn't believe in writers' block.  When you have material to write about and a deadline to meet, you write no matter how you feel, physically and mentally.  At the same time, I had writing projects of my own that I worked on in bits and snatches, and, because they weren't my "job," I had no qualms about putting them aside for months or years on end.  In 2011 I retired, and one of my goals was to complete those writing projects and start some new ones.  This has been a bumpy (although ultimately successful) process, but in the course of it I discovered several things:

1. If I start writing something before I feel "ready" to write it, the results will be garbage.  Being ready isn't a whim so much as a feeling that I know what I need to know to write this particular chapter/section.  Often this is factual information:  e.g., a mystery novel I was drafting included a  character who was a carpetbagger and profiteer after the Civil War, but I hadn't worked out how he made his (ill-gotten) money.  This hung me up for a year, until I read several books and came up with the right scam for him.  I suppose I could have written past it, but, once I'd identified his scam, other pieces of the story fell into place and the result turned out much better than I'd originally envisioned.
2. Related to 1., one of the most helpful comments I ever heard about fiction writing came from the thriller author David Morrell.  He remarked that when he gets "stuck," it's usually because he's trying to get a character to do something that he/she doesn't want to do.  If you've created characters who have a life (experiences, interests, values, etc.), they'll demand that you be true to who they are, and if you try to force them into the Procrustean bed of your plot, they'll resist.  I'm writing mysteries, and I know from the start what the solution is ... but you have to let the characters get there their own way.
3. When I'm writing nonfiction and feel blocked, the cause is usually very simple:  I don't know what I want to say, often because I don't know what my purpose is in writing it.  I was once hung up for days on what I thought was a simple thank-you note, but, when I thought it through, I realized that my real goal was to nurture an ongoing business relationship.  I taught technical writing for a while, and I used to tell my students, "If you get nothing else from this class, I want you to remember this:  Know Your Audience."  That was true with my thank-you note:  I thought through who the recipient was and how I wanted to appeal to him, and wrote my note accordingly.