Most writers get stuck at some point when they're writing. Everything is going fine and then-- bang. Suddenly it isn't going at all.
I usually get stuck two or three times while I'm drafting a novel. Fortunately I have a list hanging on the wall of things to try when I get stuck.
Here's the list, with explanations:
Print out what you've got, and read it.
Sometimes it helps to change everything to a font you don't usually use, and then print. Jot ideas in the margin as you read.
Identify where your stuckness began.
What's the story element problem?
Take a walk.
I find it most helpful to walk in the forest, or in the dark.
Draw a picture.
Don't worry if you "can't draw". No one else has to see it. Draw your main character, or any character you're having trouble with. What's s/he holding? Doing?
Don't skip this one; you'll almost always learn something new about characters by drawing them.
Make a bubble map.
Put your problem --the story element on which you're stuck-- in the center.
Game of 20.
Form your story element problem into a question. (Like "Why is Aunt Fiona missing?")
Start writing answers to the question. The first few will be obvious answers. The next few will be ridiculous answers. Keep going till you get to the more interesting answers. Go all the way to 20 answers. I usually find #17 or #18 is the right one.
Do some dialogue.
Starting from the place where you got stuck, just let your characters talk to each other for a while. Don't worry about what they say; you don't have to use this dialogue. Just let them talk, and see what you learn.