What Causes Sudden Fainting And Immobility In The Legs?

Two days ago my 14 year old granddaughter - while playing on the soccer field - fainted temporarly, and then lost the use of her legs. When she tries to walk it s very painful. The hospital has run every test and everything - everything - comes back perfect. She is a happy, very active girl, making good grades in high school, with good friends and a twin sister she adores. It is not mental. The only thing they haven t done is a spinal test because nothing indicates that we should give her one. There must be an answer ... viral, bactural, something lacking in her system? Have you heard of this before?