it happened to me before (and i have just reversed the cycle from getting worse to getting better a few days ago). All the corals closed and started to recede into the skeleton. I asked around and people said it was my additives, i was adding too many things and didnt need to. So i stopped adding, i started doing 5-15% water changes every day, used a mechanical filter which i cleaned every day. I kept this up for two weeks, lost all 3 of my mushroom corals, my frogspawn, and my feather duster, and just a few days ago my zoanthids, my candy cane, my torch, and my brain are starting to heal.
But this was when all water quality was fine and i didnt have any unwanted hitchhikers. This is more of a worst/last case scenario.