Has this behavior come on suddenly? When was she last acting active and normal?
Have you checked her crop? Check in the morning first thing - normal is for it to be empty. Check it again in the evening. Normal is full and firm.
If the crop is full and squishy in the morning, you may be looking at sour crop. If the crop is empty or has a small hard rock lump, it may be impacted.
If she's being kept from the feeder, her crop will be empty, her keel bone may be evident, and her poop may be greenish. Watch the other chickens. Are they bullying her. Does she try to eat at all?
Has she been laying? When was the last time she produced an egg? Has she been spending extended periods on a nest without producing anything? She could be egg bound. Is she holding her tail down as she stands immobile? She may be in pain from a stuck egg.
What was she doing prior to becoming inactive? Was she eating anything different from normal? Do you have a compost pile? If so, what gets put into it?
There are some clues in the things I've mentioned that can help you figure out what could be wrong. You need to do a little investigating.