I'm no expert but those that are will need more information.
Firstly make sure your hen is warm and comfortable....so bring her indoors and put her on a heat pad or under a heat lamp..... plastic drinks bottles filled with hot water in the bottom of a box and an old towel on top are a simple way to give her warmth.
How long has she not been eating? How old is she? When was she last wormed.
Read up on the common ailments like sour crop, egg binding and internal laying.
Then you need to examine your hen. Feel her crop and smell her breath, gently feel around and a little way inside her vent for an egg that might be stuck, although at this time of year it is not unusual for them to stop laying, if you live in the northern hemisphere.
Report back with whatever you find. Hopefully by then, someone with more knowledge will be along to offer advice.
Good luck
Barbara