It's hard when you hit that wall. I don't mean writer's block.
Let's say you've slaved over the first couple chapters of your novel. Mentally, at least. You know exactly what happens, exactly when it happens, and in what sequence. You write it down and it's fantastico. But then that fourth chapter looms, and you're not quite sure where your story is going.
You have scenes in mind, of course. Character P and Character V discussing birth control on a gondola at midnight. Character P's desperate race to discover who his father really is. (Hint: It's this dude.)
But what's the connective tissue? It's like thumbtacks on a corkboard. You need some red string to loop around each tack and link 'em all together. Or green, blue, purple string, whatever floats your boat. It's just a metaphor.
This is the time to step back. Review your characters. Analyze what should logically come next in the story. Fuck structure. Think about it from your characters' POV. What would you do in their place(s)? What comes next, organically? You could also do a little research. Are your characters breaking into a sperm bank? Look up some sperm banks. Are they drilling a tunnel to the center of the Earth to set off a series of nuclear explosions to restart the core's rotation? Well, if they're doing that, it sounds like you've got a summer blockbuster on your hands. I don't know what you're doing here. Go forth and rake in the five star reviews.
If you write something you are unhappy with, stop. Reassess. Is this really where you want Character A to be? Is this where Character A needs to be? Deep in space, on her space knees... praying to space Jesus? If you're writing Hannah Montana fanfiction, something has gone horribly wrong. Or horribly right, depending how you look at it.