I have this question to answer from a far-away friend, (the "ink-slinging sea gypsy") Cap'n Fatty Goodlander.
I think he's only asking something to be nice. He's already got several five-star non-fiction books to his credit, but maybe that flows more easily than fiction for him, which is what he's asking about.
Now, I'm afraid this answer is going to be mostly empathetic. I get stuck all the time, especially around midway. The exuberant blast of creativity that comes with getting to know your characters, making free and far-ranging choices, striking out fearlessly into deep water, all these usually get me at least a third of the way in, before second guessing and writer's block tries to set in. I often notice a "middle slump" in, otherwise, wonderful novels, so I don't think we're alone. I'm sure it's caused by the necessity to leave open ocean and plot a course for some particular destination, which requires tedious tacking to keep underway. (Hope my attempt at a nautical metaphor isn't too pitiful!--I've forgotten a lot from my brief sailing experience.)
Of course, people who outline don't have this problem, but it you're asking this question, you're like me and outlining doesn't work for you. I don't know, maybe I'm lazy, but I can't seem to do much unless I see one word after another, one sentence after another on the screen. I need instant reward, even if it's more like punishment.
Okay, that's the empathy part. My general remedy is to wake up early and think about what to write next before I turn on the light. I make myself do that morning after morning when I'm in the midst of a novel, and soon it becomes habit, so that I review and question myself and think of new ideas every morning. In slightly more practical terms, depending on where you're stuck, say it's only about a quarter of the way through, the character has a need and a goal, as mentioned in my "Fast Start" post, so it might be time for him or her to make a plan, or take the next step in the plan. If you're thinking in literary terms, you always want your characters to make a choices which keep the cause and effect working.
However, you can also take an occasional detour. I've heard crime writers recommend throwing in a gun. I think what that means is to add something unexpected and important, but not coincidence or anything out of character, of course. I tossed in a gun around the middle of my novel Miami Purity, but it was used for masturbation rather than shooting, which fit the circumstances. The scene runs the risk of being labeled "gratuitous sex," but I say it further develops character. It's more realistic than one might think, although I can't divulge my source.
Another possibility is to do some research. When I was writing "Stormy, Mon Amour," (in my collection Florida Gothic Stories) about a woman who fell in love with a dolphin and gave birth to a mermaid, I got stuck and realized that I didn't know enough about dolphins to continue developing the relationship. Luckily for me, (about six months later) I was able to attend a reading by Rick O'Barry, a dolphin rescuer, who visited Books and Books in Coral Gables, FL, and I learned exactly what I needed to know to finish the story. If you keep your eyes open, often Fate helps you out, but not immediately.
I guess, in general, I'm thinking that what I recommend is to get away from the computer. If you're on your sailboat, Cap'n, in the middle of crystal blue sea and sky, that might not be so easy, but heck! I can't even imagine how you can concentrate on something as tedious as writing when you're sailing to exotic ports and living your dreams.
You've probably already tried all these "solutions," so if you have any suggestions, I would appreciate them. You, Cap'n, or anybody--