What Causes Spine, Bones, Muscles Not To Weight Bear And Provide Support?

dear Dr, am sathish from india. there is a girl in my close relative. she is unable to walk but her legs are good and also if she tries to walk she fell down. in addition to that her neck is not supporting her head. she drops her head. she is moving only by her buttox. her parents visit so many doctors but all in vein. they are unable to understand what the cause or what the real problm. her age is 21. there is no perfect idea about the same. give us some idea what it may be. they are not having a history.