Perhaps you have encountered Writer's Block, that dreaded affliction where the creative muse frolics elsewhere while you sit hunched over your keyboard, numbed of mind and body, gnarled fingers poised to enter...nothing.
Paralysis has set in. You're overwhelmed by problems of plot and character that you never anticipated when you started the book in the first place. Suddenly, it's all too much. How can you decide what goes next, how can this thing ever progress, how do you get out of this mess?
Easy. Forget about the big picture. Forget about that neat idea you have to blow something important up in the middle of the book or whatever (I write thrillers). Forget about how it's all supposed to end, if you really think you know that in the first place. I usually don't, and I'll bet you don't either, beyond the broadest possible outcome like the good guys win. If you do, chances are your book is doomed anyway by the gods of predictability and boredom.
Just write a scene, any scene. I'm assuming you have actually STARTED a book, a good first step. Hopefully a scene that has something to do with your book. For example, suppose your hero is imprisoned in a dungeon. Now you're set. Description, description, description. What's it look like? What color are the stones? What kind of floor does it have? What does it smell like? Bad, I'll bet. You know, a hole in the corner for a toilet ff the hero is lucky. Are there other dungeons? Is anyone in them? Are they screaming, crying, cursing, silent? Is there a window? Is it daytime? Night? Raining? Are there rats? Bugs? What's our hero thinking? You get the idea.
Naturally scene one leads to scene two. Hero gets out of dungeon. You weren't going to leave him there for the whole book, were you? That would be depressing and you don't want to write a book that makes people want to shoot themselves unless you are seeking a grand literary prize in contemporary literature.
Okay, hero gets out of dungeon. How? He escapes by way of a secret passage, the King's men let him loose, he picks the lock, he's rescued, an earthquake knocks down the wall--the possibilities are endless.
Suddenly it's lunch time, you've just written 1500 words and it was easy. You might throw out half those words later, but so what? Write scene one. Scene two will follow.
Try it, you'll see. And let me know how it goes.