You probably have slugs. They come out at night and eat the plants. If there are enough of them they can practically eat a hosta down to the ground. You need to take preventive measures. Before spring start saving egg shells. Just as the hosta start to emerge grind up the egg shells or put them in a heavy duty plastic bag and use a rolling pin. Then sprinkle the crushed shells generously around the emerging plants. The slugs can't move across the sharp shells.
Right now you can go out after dark with a flashlight and pick them off the plants. Here are some other options.