We've all done it at some point. You're writing, chugging along happy as can be, and then one day you wake up, sit down, and think, "Damn, I don't wanna do this."
Congratulations! You've burnt out.
This is different from writer's block, if you ask me. Writer's block is 1. an excuse used by writers when they're being lazy and don't want to write or 2. when you've painted yourself into the proverbial corner. For the first one, I recommend just kicking your ass into gear. (I'll talk about that later.) For the second, I recommend thinking. Beware of the Deus Ex Machina, and think about all the ways they could get out of this mess you've put them in. Sit back and think about it for a bit. (Hang on, I'll come back to this too.)
Burning out is when you know exactly what's going to happen, exactly what you want to do, but you just don't want to do it. You don't give a damn. You simply cannot write.
Go do something else. I'm serious. Get off your ass, stop staring at the paper and/or screen, and GO AWAY. Take up a little hobby. Work in a different medium. Exercise. Let your brain rest and recuperate, recharge your batteries, and then when you feel like you can tackle it again, come back to it.
I finished writing my doorstopper in June 2010. It took me six years, mostly because of inexperience and taking breaks. But I finished it, goddammit. And then I started right in to Star. I managed to crank out about 50 pages, throwing myself headlong into the story and blazing along with all the energy of a chihuahua on crack. And then I tripped, stumbled, and burned spectacularly.
I didn't write for months. I simply had no desire to be a storyteller whatsoever. I knew I'd burned out, and I knew I'd come back to the story when I was ready. In the meantime, I read books, relaxed, sunned, biked. I held onto that little flame and I gently nursed it back to health, and then when I felt the burn I eased myself back into writing.
Go as fast as you can, if you want, but don't fear the explosion and ashes. Think of yourself as a phoenix instead.