My Personal Experience
For the past year or so, I have been working with my co-author to create a fantasy series. We worked hard and created dozens of documents detailing every character, their backstories, their interactions, and the place they have in the world. We created an entire world with five main characters and we had a rough outline that would span 5 books - not just one. There was a beginning, a middle, and an end. We had characters we loved and characters we hated and a plotline that was solidly there, following our characters through their weaving stories. We even, in fact, finished the first book and had gone through editing it once and we started drafting the second book.
It was at that point that I realized I no longer was in love with the story I was writing. While we were writing our fantasy series, I had gotten an epiphany. I had the idea for an entirely new story and I was excited about it and, better yet, it was a standalone. Suddenly, I began to notice the cracks and the faults in my 5-piece fantasy series.
Unfortunately, only you know what you want to do when you come to this realization, but here are some general pathways I've noticed that people tend to take.
1. Keep pushing - accept the fact that your writing didn't go where you wanted it to go and go back through it. Take out parts you didn't want and add parts that you feel are needed to make it the story you love again.
2. Start over - take your story and completely rework what you have. Since you know what you didn't like the first time around, you know what to look for. With this, you don't even necessarily have to start from complete scratch, just edit the crap out of it and cut it to bits. The story you originally wanted to write is in there somewhere.
3. Find a new story - put aside your manuscript and, literally, start over. New characters, new world, new genre, new feel. This can come naturally, like it did for me, or you can discover one. If your old story doesn't speak to you creatively, maybe a new one will.
Of course, this is entirely subjective and I have no room to tell people that these are their only options. You make your own story and no one can tell you what to do about it, but I've noticed that this is generally the roads the people take when they wake up and realize they don't love their story. I've heard many published authors say they feel this way about books they eventually got published. They pushed through the bad thoughts and found a way to love their writing again, but that can't be the case for everyone. Don't give up on your creativity and know that there is always a solution.