Poor baby.

Squirrel maybe?

I have a feeling these things probably have a few dormant eyes that will take over if the main stem is chomped. It might just take a while to regrow. To be on the safe side I would do a couple of things: sprinkle blood meal around the plants. Squirrels really hate this stuff and stay away from plants treated with it. You have to reapply it after a few rains. It's also a good source of Nitrogen.

To be really safe, put a couple of stakes around the stem and wrap some chicken wire around - like a little cage.

Sorry this happened.