Something terrible has happened, need help from snailkeepers

that "plasticy bit" sounds like the operculum, or the trap door that snails can close. were you feeding it? they tend to do much better with some fresh veggies once in awhile. how long did you have it?

It would be good to know what type of snail it was along with how old/big it was, as well as what it was eating and the living conditions (tank size, substrate, other creatures in the tank, water quality, etc..). Knowing more info may help someone more easily identify the problem.

What was his shell condition? Soft or hard, etched, smooth, or chipped?
Also, any clue how old he was?

my black mystery snail did the same thing, i dont understand either

Oh, one other question, is ANY salt in the water?

Pomacea bridgesii snails (mystery snails) only live for 2-4 years. If you got it from a petstore there is a good chance that it could have already been a year old. I have some snails that look even older in petstore tanks. Snails die just like fish. Sometimes it is just from a natural cause. Usually if you have poor water conditions your fish will die long before the snails. I keep fish in all my snail tanks as a precautionary measure. If I find a dead fish I know to check my water parameters.

Snails do need to be fed and cannot live off algae alone. Also, many fish medications will kill snails so if you added anything to your tank lately then that was probably what did it. The normal chlorine removers like amquel and such are fine but you need to be careful what chemicals you use in a tank with snails.

Did you balance its diet with a source of protein?