Although you haven't done anything different, something has. In my experience, bay don't like cold wind blowing on them. Another problem could be a frosty night followed by early sunshine directly on the leaves. I notice the happy bay is in shade.
I wouldn't do anything, yet, other than remove it from the cause of the problem, but when spring truly arrives, I would cut out the dead parts, feed it and work over time to shape it gently back into its original form.