I looked up somethings to try and help you. I might still be wrong but I think that if you keep it out of harms way it should heal itself. I have a friend who knows more about this I will try and get her help then let you know
If it is at the very edge of his shell, near where the snail comes out, then it should be alright. The shell will most likely grow over the crack and heal it. If it is farther down the shell then the crack won't ever heal completely because the shell won't grow there anymore. If the crack is really bad and reaches the snails body inside the shell then it could be fatal. If not then it is probably just a blemish on the shell.
Giving it a good calcium source will really help right now. Try feeding Zucchini. Some snail keepers even us cuttlebone in their tanks to raise the calcium levels.
Like rouge said it will only "heal", grow over, the cracked place if it is near the edge of the shell, other wise it will be llike a scar on the shell. And so long as the crack didn't go all the way through thenyou snail should be fine.
To build a better shell snails need calcium, and as rouge also stated, some of us snail keepers use Cuttle bones, zuccini, spinach, eggs shells, and other calcium supplements to help strengthen their shells. I have half a Cuttle bone in my filter and one sitting in the bottom of the tank, will take time for the cuttlebone to not want to float so its best to hold it down with a rock or something heavy... mine have lasted about 4 months now i need to buy a new one in maybe a month or so as my snails seem to enjoy eating it ever once in a while
Thank you for all your help !! I will do my best to get a photo of it when I get home in an hour or so, but I think the crack did go all the way through the poor thing I really don't know how he done it !