It happens to every writer at some point. The story is in your head, you've done the research, created your characters, and outlined your plot. Now it's time to get it on the page, and you can't seem to make yourself sit down and do it.
Why is that? I am a writer. I love writing. I love having written. So why is it sometimes so difficult to keep my butt in the chair?
I think I know why, at least with me. There's something wrong with my idea. Either the story doesn't excite me enough, or my characters aren't yet worth caring about, or I have too many other things on my mind.
I've found the best way to work through this problem is a three step process that usually lets me know for which reason I'm not writing.
The first thing I do is journal. I write about what's going on in my life at that moment. All my cares and worries down on the page. If that doesn't let me know what's wrong, I create a new "character" who happens to be a writer and write about his/her problem in third person. Sometimes, this gives me a new perspective to determine what the problem is.
If it's not a personal issue keeping me from writing, it might be a character issue. Can I "hear" my character? Do I like him/her? Do I want to spend the next year or more with him/her? If the answer is no to any of those questions, I know I probably need a new character. So I create a brand new character. Keep in mind, this usually means changes to the story-line, since each character makes unique choices and handles conflict differently.
To find out if it's the story, I write the copy for the back cover of the completed book. Then, I read it. Is this something I would pick up to read? Does it excite me? If not, there's no wonder I can't seem to finish it. I file all the work I've already done, since I never throw anything away, and start a new story.
Yes, I know it's tough to start over, especially if you've already put a lot of work into it, but here really is no sense in continuing to work on something that isn't going anywhere. The phrase, beating a dead horse, comes to mind. Bury the poor beast and get yourself a brand new mount. One you'll enjoy riding every day, because writing a novel is a long journey and journeys should be fun adventures.