Hi
I'm wondering if she's eggbound. Signs of eggbinding include: reluctance to move or walk, easily picked up (despite not necessarily being normally easy to catch), draggy wings, ruffled feathers, loss of appetite, white or clear discharge from vent, smallish or watery droppings, closed eyes.
Fill up a large tub with water- about the temperature that you'd wash dishes (not scalding hot by any means but not cool. Remember a chicken's body temperature is several degrees celsius warmer than our own). Fill up the tub so that when you stand the hen in it, her abdomen is submerged but she's not wet over her back. You want her to relax and stand- you'll probably have to stand with her. Do this in a very warm room- run the shower on hot water for a few minutes before bringing in your hen- if she struggles madly (usually they do not and seem to enjoy it) the you are not accomplishing anything- dry her off as best you can and put her under a heat lamp or on a heating pad.

There are a couple of other possibilities, including coccidiosis (usually bloody diarrhea), lymphoid leukosis (shouldn't have come on so quickly, but it is possible), poisoning (any compost, dead animals or poisonous plants/chemicals around?)