Your seedlings were most
Your seedlings were most likely eaten by rabbits or moles. If moles are a problem, there are plenty of mole baits on the market to repel them. Rabbits can be deterred by covering the newly planted seeds with floating row cover or fine mesh screen. Slugs might be the culprit, too. Surround the planted seeds with crushed eggshells or diatomacius earth Better luck next year!