Wikipedia, it can be as minor as a temporary interruption in productivity or as bad as a complete abandonment of writing altogether. The writer can simply run out of inspiration, or it can be that the work is simply beyond the writer's ability; I've run into that one a time or two. There are also as many coping strategies as there are writers having this problem. You need to fish around and find what works for you.
I discussed my first confrontation with this issue here. Ultimately I went back and rewrote that part, and the story continues to flow now. The interruptions are still in place, and even more have been added, to include a paid editing project I've been working on lately. So Druid Derrick may be my biggest project; it is also my longest ongoing project. But I hope to have it finished one day relatively soon.
So, do you have a block sitting right in the middle of your writing? I think it happens to all of us at some point and to some degree. I have found that the myriad of distractions in our lives can be the biggest blocks, taking the form of a type of procrastination. I know, ever since I got internet and started promoting my books and socializing, my disturbance-free window for new creation is very small. Many of my writer friends lean on music to inspire them and maybe help set the mood; I happen to find the noise distracting. My husband is one of those people that simply has to have some kind of noise going in the background. If not the radio, then the TV as soon as the generator is running. Since the TV is sitting directly in front of me, only the width of the table away, it's quite a distraction. Given the opportunity, I get up early and write before anything else gets turned on (and before my husband gets up). Sometimes I'll get up in the middle of the night when an idea won't let me rest.
I like to give my stories their own life, and a life needs attention just like any child you're trying to raise to a strong, self-sufficient adult. If you don't give this life enough attention, it will be less than it could be. Also, as a 'life', sometimes you might want to go in one direction but your story simply insists on going another way. My advice is to listen to your story. Most of the time, it really does know best. It's another way of saying 'listen to your subconscious mind'. If you're not comfortable with a scenario, change it. If you are still stuck, shoot the idea at anyone, me if you like, I'm a good sounding board. More often than not, either an idea will ring true with you, or the mere act of asking will turn on the light in a dark room and a new idea will occur to you all of your own.
My issue with Druid Derrick was something of a redundancy. At first a traditional Christian marriage that would make her parents happy, and then a druid marriage my character would see as far more binding. I did all the research. I had it all planned out, but it was like my character had simply dug in his heels and refused to go forward with it. There is also the issue of redundancy, and though the ceremonies would be drastically different, there was still the issue of getting married twice. What was the point? What was the real value to the story? Through the course of this story, it became my window into today's druid society, but though a druid ceremony would have been interesting, in truth, Derrick's wife is a very secondary character and the second ceremony would have been overdoing it. It's still something that might happen, but really I don't think so. There's already a lot of things planned for this book, and this particular issue has been semi-permanently shelved. You never know, a birth is expected, Derrick might want to cement his relationship over that. We'll have to see. At the moment, he's still learning how to socialize. He's reached a point in his druid life where he can no longer afford to hide, nor does he need to; he's a big boy now.
What is your strategy for dealing with Writer's Block. Do share; it might help someone else.