I wish there was a magic formula for this that works every time, but unfortunately, there's not. Sometimes you can get your mojo to come back, and sometimes you just have to wait for it to come back on its own.
One thing that sometimes helps me is to go back and re-read the story that I'm stuck on. Maybe not the whole thing, if it's long, but at least the most recent parts of it. That sometimes gets me back into it and makes me want to write more of it.
Also, looking back at feedback, old reviews and whatnot, sometimes helps too. When you've got people telling you they like your story and begging for more, you really feel like giving them more, and even if you're not totally inspired, sometimes that can be enough to at least make you force yourself to sit down and try. Sometimes all you need is to get started, and then the words start flowing again. But if it's like pulling teeth the whole way through, that's probably not the best solution. You don't want to have to force the entire chapter out.
This is not something I'm very good at, but it can work. If you're stuck on that one story and don't have a back-up story to turn to, try writing something new. It can be a short story, a challenge, a flash fiction or songfic, anything. Doesn't have to be fanfic or even fiction at all. Sometimes just writing something else for a change helps refresh you, and when you get bored of it, you'll probably be ready to go back to Casual.
I have a half-finished chapter of 4.15 that I need to finish, but the writing class I took this week got me really inspired to write personal narratives. I wrote one for class about my dad that I'm going to give him for Father's Day LOL, and that got me inspired to write another one about my grandparents that I'm going to give to them. So I'm working on that instead, and the pages are just flowing out of me because it's something totally different, and it's personal, so it's much easier to write.
My last suggestion is to read. Read work by other authors whose writing you admire - can be fanfic or real books. Don't just pay attention to the story, but also the writer's craft... the way they use words to tell the story. Sometimes that can be very inspiring, when you think to yourself, "Hey, I could do that too," and then you want to go do it. It helps if you read something that's similar to your own story, at least in genre, but not always.
Hope this helps!
