It could be that you just need to start writing, but also, maybe you need to work on subplots and depth to reignite your excitement. For me, I do tend to lose interest in stories when I prep, but then suddenly I'll find some additional storyline or some different reason for something to motivate the characters or whatever and suddenly, I'm excited again.

If I feel like I'm bored with the story and/or characters, I'm go to the NaNo forums and check out the Adoption Society threads for ways to reexcite the story and my passion for it. But that's me.