It's referred to as Mantle Collapse; the only reasons I've heard for snails dropping their shells are disease (illness, parasites, etc.), injury, and possibly somethings like toxins or environmental conditions - in almost every case I've heard of, the shell dropping off is an indication that the snail is going to die soon.
That said, I have heard that Mantle Collapse can be healed with land snails (though this is highly disputed, as people dispute if these cases were truly Mantle Collapse, or if they were cases of Mantle Tears or Puffy Mantles), but I've never heard of it being healed with aquatic snails.
The cases that I've heard of recoveries with:
https://petsnails.proboards.com/thread/18414/positive-outcomes-mantle-collapse-updated
Edit: Looks like the FB link failed, but here's the info of the article:
"MANTLE COLLAPSE IN SNAILS
GEORGE SEBASTIAN NEUFELD·SATURDAY, JULY 22, 2017·"