My 85 Year Old Mother. Was Using A Walker, With

My 85 year old Mother. Was using a walker, with a few months she can not use her legs at all. They give her lots of problem at night jerking, pain etc. We have been to a neurologist, had a spinal tap. When to a spine specialist her spine looks fine. why would she just loss the use of her legs. ??? Something had to set it off, The Drs we went to, do not know why.