The only way to repair it it to get another piece of shell or some eggshell and glue the shell to the snails shell. No adhesive of any kind can touch the snails body.
Here's a good site that talks about shell damage and how to repair it if needed.
I'm not sure if you could use silicone instead of the epoxy they talk about on this site.
35G barebottom: 2 boesemani rainbowfish, 4 congo tetras, Low light plants (1.5wpg) attached to or planted in my own handmade ceramics - Anubia v Nana, Anubia v Barteri, Red Rubin Sword
2.5G - Spot (beautiful betta - Soft pink with red spots on his fins. Java Moss.