Unless you're Chuck Palahniuk, it'll happen to you eventually. Probably quite often. You'll sit down to continue working on your piece, and you can't get going.
You'll be halfway through, and it's coming along swimmingly, and all of a sudden it's like you can't form a complete sentence.
It's frustrating as hell, and more than a little discouraging. It's called writer's block, and it happens to just about everybody. There are few worse feelings in the world than the one that comes over you, when you realise that you're stuck.
There's no rhyme or reason to it. At no point in your story can you see it coming, and no point in anybody else's story where you can see that it happened. It just does, and it kicks you in the balls.
A lot of people have their own cures for the affliction (and affliction it is, make no mistake; I'd pay my entire life savings to the doctor who finds a cure for it), and while some work, most don't. I've tried them all. I've tried starting a new story, I've tried writing about nonsense, I've tried reading a book, and I've tried forcing myself to continue regardless with the plan to fix it in editing.
It doesn't work.
Sadly, the only thing that works for me is waiting it out. Sometimes this means a project will sit idle for a few days, sometimes it's months. There's nothing for it.
The flip side of this, however, is that when it comes back to you... oh, when it comes back....
Some of your best work will be in the furious few days after you get past a block. It's almost as if the creative juices have to build up, past their normal levels, and explode in a torrent all at once before subsiding back to normalcy.
So, be patient. It'll happen. You'll get out of your slump, and put down some of your best work. Which doesn't help your mood when you're in the middle of that funk, but it makes you feel a whole lot better about it being done.