Your hen is brooding, and she may sit that way for a month, or more. You can either wait it out, just make sure she gets food and water. I take mine off the nest myself a couple times a day, and they scratch around a little, or maybe get a quick drink, but then it's right back to the nest. Brooding is a common occurrence for some hens. Some breeds are more likely to brood than others.
You can try to break her brooding by removing her from the rest of the chickens and putting her in a cage with no nesting materials (preferably a wire bottom) for a few days and then let her go back in the general population. If she resumes normal activity, you have been successful, and if she runs right back to the nest, repeat the process until she is "broken" of the habit.
Good luck. There is lots of good information about brooding on this site and others.
Sally