Did you treat them all before with Corid? Hopefully, the hen got pushed off as you said, but I see one occasionally who is busy eating when the light goes off, not making it up on the roost in time. Just check her out tomorrow. If you do see another bird who becomes lame or is acting off, just find out how to go about getting a necropsy. That way you will have a plan and know the costs involved if one is necessary.