We've all heard way too much about this mythical ailment, mostly from the movies, because 1.) people who write movie scripts are writers and are writing what they know, and 2.) as has been pointed out repeatedly, writers actually writing aren't very interesting to watch; the "block" is the only way to get them out of the house.
That said, I don't think "writer's block" is much different from any kind of procrastination brought on by doubt or fear. In my case, anyway, my reasons for not writing usually boil down to thinking it's not going to work. I don't want to spend two or four or eighty or ten thousand hours on something that's going to be bad, do I? In the same way, I don't want to cut back on carbs because it probably won't make me feel better; I don't want to see a movie because I might not like it; I don't want to go grocery shopping because I'm afraid of the parking lot; I don't want to work because I'm lazy ... or, I mean, it might not be satisfying.
Still, in most of these cases, I do the thing anyway. Sometimes, as with work, there's a concrete financial incentive. But other times I'm able to get past my objections relatively easily. How do you know it won't work if you don't try it? I ask myself, and then away go the carbs--at least for a little while. But with writing the resistance seems greater, and I can only think that, unlike these other issues, my ability to write says something fundamental about me. Writing something bad means I'm not really a writer after all, which means the only certain way to preserve the illusion that I am a writer is not to write.
So how to combat this? Maybe by remembering that writer's block is the ultimate absurd condition--and just writing any goddamn thing you can.