WInter burn can indeed cause some discoloration of boxwood and other evergreens, but if the discoloration is severe and the twigs (not just the leaves) are brittle and dry, you may have a worse problem than a little burning.
Sometimes if shrubs are not well-hydrated going into winter (with its windy but often sunny days that warm up leaf surfaces, followed by extremes of falling temperature…havoc!) they can suffer substantial damage or even be killed.
Do NOT feed right now; and do not prune…yet. Let the plant tell you what the extent of the damage is (you will see where/if new growth starts to push). You may have to wait a few more weeks to really be clear what is dead and what is alive.
What you must do at once: water, and not lightly. Make sure the entire root zone is thoroughly moist.
And then you watch. Hopefully, the twigs are not brittle and you don’t