Rabbits would be my best guess. It should sprout again from the base. I wouldn't move it, but I would maybe put a cage around it to keep the critters away!

Could be deer too. Some deer don't eat them but some do.

I've never seen a rabbit around here, just squirrels, chipmunks and a woodchuck. No deer either, although there are some in the woods nearby. This clematis is in the very middle of my mound garden and it's highly unlikely that a deer could access it without trampling down a lot of other plants and leaving some signs of its presence. Glad to hear the roots might be OK. I'll just mark where it is and see if it comes back.

If you have squirrels, chipmunks and a woodchuck, you can be sure of having rabbits, especially if there are woods nearby. Having said that, I did catch my cat chopping off my clematis in the same way. I put a wire cage around the base and that kept everything away. Your plant should grow back from the base.