Thank you for your question.  Although we are reluctant to diagnose a problem without photos, I suspect that your plants have the soil disease described here, caused by poor drainage:  https://pnwhandbooks.org/plantdisease/host-disease/boxwood-buxus-spp-phytophthora-root-crown-rot

Only the Monterey Garden Phos is available to homeowners, and, when used as a soil drench per label instructions, may lessen the problem.  However, once the plant tissue is dead, it can't be revived.  Sorry, and good luck!