There isn't enough info in your post to really say anything - I would suggest you go to the instructional post on the emergencies/diseases/injuries, read it and then make a brand new thread.
That instructional post will tell you to explain lots of details about your hen - how old she is, if you have examined her for lice or mites, if she is willing to eat, if she can stand, if her eyes and nostrils are clear, what her vent looks like, if anyone else in the flock isn't feeling well, if she is laying, if her vent looks nice and pink or pale and puckered, if you have felt to see if she is egg bound, if you have ever wormed her or the flock, what her droppings look like.......
would like to help you but it is as if you posted a pic of your 6 yr old kid and said tommy is sick, help! we just need to know so much more to begin to help.
in general, isolate her, see if you can tempt her to eat, do a thorough exam to see if you see any injury, swelling, mites, lice,.....and post a new post. Hopefully someone will be able to help you. And, by the time we notice something is wrong with a chicken, they are often very ill because they hide their illnesses.
best of luck and keeping my fingers crossed for you