I used to say I didn't believe in writer's block but with age and experience comes the wisdom that just because it never happened to me doesn't mean it doesn't happen or isn't very real to someone else.
Most cases I have heard of have to do with a kind of hitting a wall, usually of the writer's own making, a wall of fear. A big advance, an unexpected mega bestseller, an honor that feels undeserved, or deserved but...
What if I can't do it again? What if that was all there was in me? What if that's all I had to say? What if I don't really want to do that again? What if I can't deliver what people are expecting? How can anything I do merit that kind of money, praise, recognition, anticipation? It doesn't have to be a huge thing, of course, it can be just knowing your critique group thinks your present work is "IT" and wondering - how do I write up to that?
If you dwell on that, I can see how it would cause you to freeze up. But if you have the ability to build a wall strong enough and scary enough to cut you off from your creative drive, you have the ability to take that writer's block and build on it.
The key is to get those words, to do SOMETHING, to WRITE! Scribble in notebooks and tell yourself no one will ever see it, if that helps. Talk your story into a digital voice recorder. Blog it. Put it in a file marked "practice" or "just stuff". Tweet it out if it comes down to that.
Every day people do things they think they cannot do. I mean tough stuff. This is just writing. Just words. Slap some down, then some more, add a few more and when you have enough to go back and work with, fix them up and move on. You can do it, if you get out of your own way.
Says the writer who has been avoiding writing for 2 weeks. Okay, here goes... chapter 6...