The water could have gotten contaminated (was it covered?). If the wipeout occurred immediately after, then yes. See if the snails show up because their death may lean toward some sort of non-disease related problem such as contamination. Also, your live rock should be very much alive if all is all is well in your tank. The creatures living in live rock, such as worms, sponge, bristleworms, spaghetti worms, brittle stars, etc. should have exp die-off as well if your problem is a system contamination. In the meantime, do water changes. Never leave change water wo a pump, and always cover the change water container. Salt should be mixed no less than 2 days before using, with pH and salinity tested to sure that parameters match your aquaria.
Leave your tank fallow for 2 mos without fish. In the meantime, check out your tank routinely to be sure that other creatures are doing well in the aquaria. You can get some inverts in the interim but nothing with hardshells. Anything you wish to add that is hard surfaced, QT first for 2 weeks.
When you do start getting new fish, be sure to QT no less than 3 weeks. More if any problem arises.
Were the black spots on the NASO bumpy like salt, or just a discoloration of the skin?