Hi Chris and welcome to the forum .
I'm sorry that your first post here is about
tragic and a sudden loss of discus .
While I never had it happened to me I've read a lot of similar stories here on the forum about sudden death of healthy discus and they are always explained with the "Dash and smash" discus behaviour .
They do get easily spooked from a sudden light change or an unexpected movement in front of the tank and as a result of that they sometimes end up banging hard on the glass which results to death .
I personaly can not think of any other reason or condition or disease that can kill only one healthy looking discus in a matter of hours .
Heads up Chris and try not to worry too much over this accident.