Sorry to hear about your hen.
There are many possibilities that can cause this type of behavior, so please answer some questions for us to help narrow it down for ya..
You say small hen... do you know what breed, bantam? How long have you had her and where did she come from? What do you feed on a regular basis, including treats and supplements? What is the size of your flock and their coop/run? Do they free range beyond that? Any recent additions or losses? Anyone vaccinated for anything? Did she ever need to be treated for coccidiosis when younger? Any recent chemical spills like leaking lawn mower? Have you checked the feed for accidental mold? Can you here any raspiness in her breathing? Can you tell if her crop is full, hard, squishy, empty? Any sour smell coming from the beak? Are eyes open, normal color, and round pupils? Have you seen any droppings she has passed recently? Please report how they are. Have you ever wormed or had a fecal float done at a vet to see your internal parasite load and species count? Where was she when you found her on her side... nest, coop, run, yard? Do you know when her last egg was laid? When was the last time you saw her eat or drink anything? Is she molting at all... or sporting any angry red skin under her feathers below the vent area? Can you tell if she has any control over her legs or wings? Did she respond like pain when you examined her or just general annoyance? Did you feel any heat or swelling when you examined the legs/feet? Is she able to hold her head upright when you are holding her upright.. or does it hang? Any other recent changes like bedding, lighting, or heat? Sorry so many questions!
Some folks find a chicken sling useful...
Quick common possibilities I am looking at... nutrient deficit (or imbalance), slow or sour crop, Marek's. Please don't panic or jump to conclusions... many times these things can be ruled out. Please post any video or pic that might be helpful, including poo!
Hope she recovers quickly!
ETA:
@Andrew10
