Write. It all starts with a word. You put one word onto the page and then another. You follow that word with one more, and then one more, and one more. Soon enough there's a sentence. Then another sentence. A paragraph. A page. A scene. A chapter. And the more you write the more the characters come to life. The more the plot is revealed and the setting explored.
At first you are driven by sheer inspiration and force of the initial story. Then suddenly your running off of wisps of energy left by that inspiration and more of a desire for the characters to make it to the end. Eventually you're left with fumes of that desire, and the only fuel you've got left is pure force of will to finish that damn story. But even that doesn't last. That will, indeed, run out too.
Now you have, what? An unfinished story with no will or desire to really finish it? Oh, but what about that one scene? You know, that one that comes later in the story. The one you were really excited about writing before. You aren't there yet, and there's a good amount of slog to... well, to slog your way through. (Slog is a fun word.) And that just doesn't sound exciting. In fact, it seems irritating, boring, and you generally just don't want to.
But you should.
That's right. You should. Despite all of that complaining that you're doing right now, you should write. That scene you're excited about? You'll get there. Sure, you'll have to drudge through however much slog is separating you from your scene. It may seem uninspired to you as you write it. Hell, it may even sound like shit as you put the words down. But at some point, if you keep going, you'll get back into it. Whether it's at your fancy scene you were excited for, before that, or even after it. If you keep going, you'll find yourself getting back into your story.
Besides, if you do anything with that story of yours, there are going to be how many rounds of editing? It's gonna get read, edited, read, and edited again. It'll be polished. That's what revision is for. It might even have been alright as you were writing it, you could have just been too focused on slogging through you didn't pay too much attention to what you were putting down. And after all that editing (not just on that part, but the whole thing, mind you), readers will not be able to tell the difference from the glorious, inspired parts you puked rainbows you were so excited for and the drudgery you slogged through with no hope of finishing. See what I'm saying?
So write. Stop complaining. Stop procrastinating and making excuses. Just stop. Write.