A new idea can be like crushing on someone. You're all excited and can't wait to round every bend in the journey. Then the initial thrill fades. Eventually, you may get to a point where it feels like just work. You're trudging through page after page, but the excitement you had at first is completely gone.
I had this experience with Dead Locked. For a time, I lost all interest and considered moving on to another project. As you can probably tell, I managed to not only get back on track, but get that first rush of excitement all over again. What can you do if a project feels like it's sapping the life out of you?
Take a short break to get perspective. Sometimes even a day or two away can reignite your desire to finish. I regularly walk away for a short time (emphasis on short) to refresh my creativity and see things in a new light. Word of caution: don't walk away for too long or you may not go back to it!
Reevaluate the direction of the story. If you don't like where the story is going, or you feel like you're shoving a square peg into a round hole, you won't want to write. I had this problem with my current novel. Somewhere in the process, I lost track of what I like and went off on a tangent I didn't feel passionate about. Stick to ideas that you love, and you won't lose motivation.
Get an objective opinion. I got pretty discouraged at one point when my book seemed to be spiraling downwards and I ended up just telling my sister everything I felt about it. Her outsider insight jolted me awake to the problem. Don't be afraid to confide in someone if you feel down about how things are going. They may say something that sparks new desire to push forward.
The important thing is that you don't give up on your story. You've worked too hard to throw it away!
Have you had this experience with a story? What did you do to keep moving forward? How did you get excited about the project again?