I recommend a vet as well, but that's not always possible or economically feasible. You'll want to make sure you examine her carefully - check her feet, esp the bottoms, turn her on her back and fold her lower leg up to her body, feeling the hock joint while you do this. You're feeling for grinding or popping or anything unusual. Feel if the muscles are withered or otherwise abnormal. If her toes automatically curl when you bend the leg up towards the body, it is possible there is a slipping tendon.
I don't know of any leg ailment offhand that might cause a weakness, but I read about something that might help in a chicken book (I think it was Hen and the Art of Chicken Maintenance, but I am not sure). The author had a chicken that went inexplicably lame, I think in a similar fashion, and he constructed a sling/hammock for her, so that she could be off her feet if need be (the hammock let her feet touch ground, but was high enough that she could hang limp with it supporting her weight if she chose. Her food and water were handy, and there was a "poop hole" cut into the fabric for, well, poop. I have used the hammock/sling system for smaller birds & chicks with great success (in smaller birds, it is more like a "chick chair" that you make with a container or box to hold the sling in), and if you go that way, I recommend: 1) some high-protein treats to boost the muscles, like low-fat cottage cheese or scrambled eggs; 2) pin a piece of fabric across your chicken's back while she's in the chair/hammock, so she can't struggle out of it - she probably wouldn't be able to pull her legs out & thus would be stuck hanging upside-down; 3) put food and water where she can reach it in the chair/hammock; 4) Take her out of the sling a few times a day and massage her legs, let her get some exercise & try to use those muscles, so her legs just don't fall asleep in the chair/hammock all day, and 5) make sure she is warm. When my chickies are sick or injured, warmth is not only soothing but it also lets their body burn its precious calories on healing rather than heating. Somewhere around here is a thread on spraddle leg that talks about chick chairs. I think I just looked up chick chair in a search engine - on a bigger bird I guess it would be more like a hammock in a rubbermaid, but the concept is the same. Hope this helps!
