While I do not believe in writer's block, I do believe in writing speed bumps. There are points in the process where you're speeding along, where your fingers can't type fast enough and then all of a sudden, something in the road makes you slow down. Whether it's a plot problem, a character flaw, or the uncertainty of what comes next, there are numerous bumps throughout the journey that cause you to back away from the computer and think.
Every writer I know handles this stage differently. Some take a walk, others do housework, some read or do a crossword, while others simply stay in the chair and stare at the screen. I personally think that stepping back and letting the problem marinate a bit is healthy, but when does it become an excuse rather than a productive break? When are you simply avoiding the problem rather than working towards a solution?
I think the first step is identifying your habits. When you hit a speed bump in your writing, what happens? Do you suddenly remember a phone call you have to make? Do you look around and realize how cluttered the house is? Do you check your e-mail, then your facebook, then you're friend's facebook, then their friend's website, then click on a YouTube link from that website and so on? Knowing your usual attempts to run from a writing problem is the perfect way to avoid them. While I truly believe that taking breaks can be productive, I also am a firm believer in staying in the world of your story and that when you pop out of that world, you are avoiding the problem.
I cannot speak for other writers, so I will use myself as an example. When I hit a point in the writing that I need time to think and brainstorm, I cannot sit at the computer. So I do some laundry, wash the dishes, go for a run or a swim, anything that is quiet and that I can stay in my head. If I surf the web, I'm back in reality. If I read a book, I'm in someone else's fictional world. I strongly believe that, when you're writing, you should stay in the world of your story even if you need to step back from the computer. But our minds wander, it's inevitable. I'll be unloading the dishwasher thinking of how I can kill one character without another character knowing, and I'll see something or hear something that sends me on another train of thought. That's when you have to bring yourself back to your world, to the task at hand. Otherwise, you're not writing, you're just doing dishes.
While it may work for some people, I cannot understand how anyone can read a novel or watch TV as a way to get inspired or fix a problem in their story. Tasks that require lots of thinking and concentration generally don't work either, it takes your mind off your story. Identify things that inspire you without distracting you. Listen to music, take a walk, do some research, something that will keep you thinking about your story while letting new ideas and possibilities enter. Checking in every few minutes to make sure your mind hasn't wandered too far is usually necessary to keep focus.
I understand that this whole concept seems weird and vague, but I really think it's something that every writer struggles with. While we're into the writing and we're on a roll, there's nothing anyone can do to pry us away from the keyboard. But in order to have those spurts, it is necessary to plan and hitting bumps in the road is inevitable. The time spent away from the computer can be just as important as the time spent actually writing. Identifying the common distractions is the first step to avoiding them. That way, when you're significant other comes home to find you doing the crossword, you can honestly tell them that you're writing.