Is she on nest day
and night?
She could be broody.
She'll get off eventually to eat drink poop dust bathe usually once a day.
You can take her off nest and dip her beak to prompt er to drink.
Don't leave food/eater that she can reach from nest, she needs to get up.
You'll need to decide if you
want her to hatch out some chicks, and how you will 'manage' it.
Do you have, or can you get, some fertile eggs?
Do you have the space needed? She may need to be separated by wire from the rest of the flock.
Do you have a plan on what to do with the inevitable males? Rehome, butcher, keep in separate 'bachelor pad'?
If you decide to let her hatch out some fertile eggs, this is a great thread for reference and to ask questions.
It a long one but just start reading the first few pages, then browse thru some more at random.
http://www.backyardchickens.com/t/496101/broody-hen-thread
If you don't want her to hatch out chicks, best to break her broodiness promptly.
My experience went like this: After her setting for 3 days and nights in the nest, I put her in a wire dog crate with smaller wire on the bottom but no bedding, set up on a couple of 4x4's right in the coop and I would feed her some crumble a couple times a day.
I let her out a couple times a day(you don't have to) and she would go out into the run, drop a huge turd, race around running, take a vigorous dust bath then head back to the nest... at which point I put her back in the crate. Each time her outings would lengthen a bit, eating, drinking and scratching more and on the 3rd afternoon she stayed out of the nest and went to roost that evening...event over, back to normal tho she didn't lay for another week or two.
Water nipple bottle added after pic was taken.