Possibilities include rabbits, squirrels, deer, and woodchucks. Dogs sometimes eat the flowers, but generally leave the stems alone. Tortoises may also cause damage but probably aren't big enough to reach the top blooms.To try to help the plant recover, keep it watered and in the sunshine. Use a water-soluble fertilizer once a week.
To help prevent further attacks, consider spraying the plant with an animal repellent. (Ask at a garden center for recommendations.) You'll need to respray each time it rains.
You may also want to watch and see if you can figure out which furry critter did the damage. For many, just moving the plant up on a stand or shelf may be all that's needed.