I don't have snails, but I have read that cracks in the shell are indicators of a lack of calcium. Is your water soft? How often do you do water changes?
you can HELP it by adding calcium to your water. calcium carbonate or calcium carbonate will both work. depending on the cause/severity of the cracks, it may never be completely fixed, but adding calcium will help prevent the cracks from forming.
how big is your tank? how many snails do you have in your tank?
I believe Griffin is correct.* Adding a calcium supplement to the water can help it heal.* Hard water is supposed to make it easier for their shell growth.*
I think filling the tank with milk would be a better source of calcium. : seriously, that link of Gunnie's should help out the snail in healing up as much as it can.