Chicken suddenly stopped walking or standing! Help!!!

[ReflectiveSeasons]

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My chicken doing to same thing

I have a chicken with these same symptoms - 5 months old, acts content grooming herself and moving her head around, but does not show any movement nor reflex in her legs at all. Has been this way for 4 days. Any idea how to treat, and if she overcomes an illness or whatever the cause, how likely is it she will regain leg function?