It could be a rabbit looking for new green shoots. If the top inch of the lily growth tip is gone, they may grow out of the ground more but will not bloom this season.

Best thing to do is leave them where they are, let them go through their normal cycle and in the fall put a chicken wire cage around them - that way they're protected as they first emerge next spring.

If it's not rabbits first thing in the spring, it's deer later on that eat all the buds. I both cage and spray against pests.