A bunch of things love Cannabis seedlings.
Cutworms will eat through the stem near the ground, while a slug(look for telltale slime trail) will eat both stem and leaves, but rarely eats the entire sprout.
I'd bet it is one of those two things.
Cutworms can be controlled with Bacillus Thuringiensis, also known as Bt or Caterpillar Killer by Safer.
A better idea is to start your seeds in small pots, away from garden pests, and put them in the ground when at least six inches tall.
Good luck!