that really sucks, sorry to hear that...you have/had a beautiful tank and hopefully it'll flourish again one day
on a similar note, i, myself, just averted a tank crash...nowhere near as quick as yours, however...my tank's decline started probably over the course of a few months and gradually got worse and worse...it started with my very large monti setosa STNing like mad then other montis followed then acros started showing burnt tips followed by them STNing as well...all in all i lost quite a few frags and 1 or 2 colonies i've had for years...it turned out to be potassium overdose and, unfortunately was one of the last things i tested...at first i thought it was ULNS mixed with high alk and lighting because of the burnt tips so i did everything to get my nutrients up and alk down but things kept dying...what fixed everything was getting that K test kit and then doing several larger than normal water changes and my tank rebounded really quickly
perhaps your issue is something you don't normally test for? i have no idea if you dose anything but i'd been dosing K for over a year and never had any issues (and absolutely phenomenal colors) until it built up so much that it started having some real ill effects on my livestock