My mare continuously went down, and seemingly had little control of her hinds legs for very long, vet guessed that she had a pinched nerve in her back. She was put to sleep within hours of her symptoms starting.
I'm sorry you are going through this - it is so much harder when there are not certain answers for what happened, or what could have been done.
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan