I can think of two possible culprits for your failing hostas. Perhaps they accidentally got hit by overspray with a broadleaf weed killer that was applied to your turf. Or perhaps you've got voles. Voles generally travel in underground tunnels like moles, but they love to raid perennial beds and devour the crowns of perennials. Dig around a little where the hostas were and look for an open cavity where the crown used to be. If you do have voles, you can should be able to drive them off by lightly sprinkling bloodmeal onto the ground around your plants.