It's possible that all may be lost; on the other hand, maybe not. Distinguish between losing interest in the project and merely losing interest in its current state. The latter is somewhat more salvageable.
- Do you remember what, in your initial writing, you found exciting about the book? That's the key, because if there's anything to go back and save, it's got to have that spark of enthusiasm. Usually, once you work with any enthusiasm on a piece, you'll have several such sparks - some that kicked the story off to begin with, and others that popped up along the way, things you were pleased with or that surprised you as you wrote them.
- Why do those elements no longer excite you? Sometimes your enthusiasm wanes as a once-exciting idea grows familiar, even if only to you. You might try to recall how it originally seemed new and interesting, but if this doesn't give you the boost you need, you may be burned out on the idea. On the other hand, if you can put your finger on particular problems - "I can't pull off these intense dialogues well," "I chose a stupid location and now I don't like it," etc. etc., then those are specific problems that can be overcome. You might change those elements for something more to your liking, and that would get you past the hump and back into writing something you're interested with.
- What could you change to make the story exciting again? Sometimes you can make a significant twist that breathes life back into your story. Orson Scott Card describes several cases where a plot or a setting lay dormant for years until he ran against a character or idea that woke them up again - like "Speaker for the Dead," which suddenly worked for him once he inserted Ender as its protagonist. You could try to do this actively. Does your story sound more enticing if you shift it into space? Into an underwater prison? If you tell it from the bad guys' point of view? You've got a strong but uninteresting backbone. Often, if you put down your author-spectacles and put on your reader/reviewer-glasses, you can say, "Well, this story would be pretty boring, but what'd make it cool is if he'd done..." and you're off again. I do this all the time for books I read and TV shows I watch - and when I need to, I can do it for stuff I write, too.
All that said, it's also quite possible you're burned out on that particular story, and mining it for your lost effort might be unproductive. You're the only one who can really tell whether you've still got any interest in your original work. I think a good guideline is this: do you have anything specific from the original piece you want to save? Or do you just feel bad about the general expenditure of effort and time that won't bear fruit?
If it's the first, then you know what you're looking for, and that's something you can probably save. If it's the later, than it's painful, but you probably shouldn't bother. Think of it this way - you've got a thousand other plots that are equally available to you, from books and movies and TV shows you've already seen. You might as well cannabalize one of those as your own. Why force yourself to write to a plot you're not interested in? Chalk the effort up as good experience, and start on your next project. If you ever regret that choice - if you ever do see something in your initial work that you want to use - then hey, you can always dust the file off and go right back to it.