I have this big hole right on my chin-seems to have come out of nowhere.
I understand it is caused by elevated cortisol levels due to stress (I am definitely stressed!)
Is there anything I can do to bring the hair back and/or prevent future loss?
Yes, its definitely from stress. Stress does cause hair loss, especially its is constant stress.
I can tell you that from my own experiences.
Have you seen a Derm?