Chook not eating :(

when did she last lay an egg? she may be trying to pass a soft egg - this will make her feel unwell.
What is her comb like? is it pale and floppy or tinted purple?
Does she have a temperature? feel under a wing then with the same hand feel under another chickens wing to compare.
What is her crop like first thing in the morning and then later in the day? Is it empty or full first thing? Does it get bigger during the day?
Is she snuffly or bubbly eyed or sneezing?

I had one like this a few weeks ago, not exactly ill, but not right either. Brought her indoors to keep an eye on her and sadly she died. She had been 'not right' only for a couple of days, still drinking and the occasional peck at food. Her comb remained red with only a slight floppiness about two hours before she died. She had been the one who laid my biggest eggs. Looking back she probably hadn't laid for about a week, there was no evidence of any problems with broken eggs (I very gently checked) but then the other three were laying very intermittently since late Sept anyway which I put down to the weather change and moulting.
Interestingly since she died the others have gone back to laying 3 eggs per day.

Don't want to depress you with my tale, just hope your girl gets better soon