Horse unable to move?

my horse suddenly is unable to move her back legs, she is 18yrs, the vet came out and said it is neourological and she needs to be put down, he said she is in no pain, a vet is coming out wed. to put her down, has anyone heard of this before, are they jumping the gun to put her down, my 5 year old granddaughter is heart broken, she has been riding her since she was two, she can ride her in arena by herself she will miss her.