What can I do to save it?
Deer Ate My Rhododendren Down to Nothing
Certified GKH Gardening Expert
About all you can do is prune it back and it should regrow as long as the roots are still fine. Other than this, you may need to do something about the deer. This article might help: https://www.gardeningknowhow.com/plant-problems/pests/deer/deer-repellents.htm