Either you had a Ammonia spike or someone put something in your tank.....tanks dont just die like that overnight unless they get poisoned or they dont have adequate Oxygen. All typical salt params except for Salinity wont do anything to that magnitude that fast either. Given you didnt have a power outage from what you have said, I would say the most likely is that something died and spiked the ammonia.....
But certainly people have been known to do shenanigans and poison tanks too....
Unfortunate either way.... sorry man
