The further you get into a novel, the more likely that the essence of your story will hide from you. Side characters want to tell their stories. Main characters wander into dark alleys. Outlines and note seem to be written in a different language.
Some sort of shock treatment is in order.
My first resort when this happens is to return to my List of Ten for the book. I almost always find a potent reminder of why I wanted to tell the story to begin with. And, more often than not, I add to the list before I leave it, renewing my enthusiasm and focus.
Sometimes I interview my character or write him/her a letter. This get the voice of the character back into my head (and often the problem is shutting it up).
Recently, I was teaching my Flash Fiction course, and we got into a discussion about setting. With Flash Fiction, setting can make or break the story, and my main point was to think of places where interesting things happen. Some obvious ones are police stations, hospitals, weddings, funerals, and battlefields. But, if you think less dramatically, you can think of places and situtations where conflicts occur over mundane things. In the class, we talked about people trying to renew their drivers licenses, buying suits for teenage boys, blind dates, and so on.
Make the list long enough, and you're apt to react strongly to one of the places or situations. You'll feel that nibble of inspiration. The next step is to put someone into the place or situation who surprises the audience. We workshopped an emergency room in the class and ended up having two people arrive who had been sewn together -- arms, sides, hips, legs, and feet. The didn 't get along, and their stories on how this happened didn't match.
What we ended up with was drastic, but entertaining.
Think of how you might take this route to get to the essence of your story. What is the place or situation that will put your character into the most uncomfortable position. Can you include an element that will surprise your reader? What conflict could push it as hard as possible? Could you write this as a Flash Fiction (1000 words or less) story?
If so, your story will come back to life, and you'll have a reference point as you go forward. You also may learn more about you character and what your story will be.