First, realize that this happens to everyone, even well-known, experienced authors, so don't beat yourself up over it. It doesn't mean you can't write. What it may mean is that you're striving for perfection, and worrying that you won't achieve it. Your writing, especially in a first draft (and that's what NaNo novels are--first drafts), doesn't have to be perfect. That's what editing and revisions are for. The important thing is that you just write. Get the story down on paper. You can go back later and correct spelling, grammar, punctuation, plot discrepancies, etc.
Walk away. Don't give up, but do step away from the keyboard for a while and do something else. Watch a movie, take a walk, call a friend, jump on the treadmill or stationary bike, work in the garden...anything to get your mind off writing for a little while. Then, after you feel refreshed, come back to the writing with a new perspective. Maybe something you've seen, heard, smelled, will trigger an idea for your story.
Write about it. That may sound contradictory since writing is what you're having trouble doing, but sitting down and writing out why you think you're feeling "stuck" might just get you unstuck. It's like talking out a problem with a friend. They might not have the answer, but you'll feel better because you've gotten it out in the open. And, sometimes just talking about it will suddenly give you clarity about the solution.
Try doing a writing exercise. This can be as simple as going to Writer's Digest's online writing prompts (http://www.writersdigest.com/WritingPrompts/), picking several random words from the newspaper and writing a story using all of them, or just writing down whatever comes into your mind (stream of consciousness writing).
Remember why you started writing in the first place. NaNoWriMo is a challenge, but it's voluntary and it's supposed to be FUN! Go to one of the more upbeat forums, such as the Nanoism forum, and let laughter recharge your creative batteries. And remember, the world won't end if you don't hit 50,000 words by the end of November.
Try to stick to a routine. If you write in the same place, at the same time, every day, your mind will begin to make the connection that when you do certain things it's the signal to start writing. Some writers even have a ritual they go through before actually writing, perhaps getting a cup of coffee and putting on a favorite item of clothing, playing a favorite CD or turning on the radio, anything that will let your brain know "this is writing time."
Jump to a different part of your story. If you're stuck in the middle, try writing the ending, then go back and fill in what needs to happen to get you to the end. Add a new character to the mix to bring in fresh action and dialog. Or if you're having a hard time with the beginning, start with the middle. If you haven't done an outline, try writing one now so you know what has to happen to get you where you want to end up.
Breathe. Try some relaxation exercises such as deep breathing, meditation, yoga, qi gong. Put on some soothing music, close your eyes, and picture a scene that is relaxing and calming for you.
Read. Grab a Coke, some coffee or tea, and a good book and read for a while. Reading what someone else has written may give you an idea you can use in your own story.
Just write. Even if you write a nursery rhyme, copy something you've already written, write a page of "It's your fault," "No, it's your fault," "Is not. It's your fault," etc., the important thing is to let your Muse know you mean business. You're there to write, you're writing (even if it's gibberish--you can delete it later), and that may get your creativity flowing again.