Most likely they dried out too much, specially if they were potbound. Nothing you can do now except to keep them well watered and keep your fingers crossed.

Sometimes a very weak solution of white vinegar--1 ml per liter of water--plus some seaweed extract, will help the roots to grow back faster, but in my experience, Boxwood isn't very forgiving.

They've had it. Forget and move on. New ones, JI compost if in containers, and don't forget to water! It's been an odd year, and even when it has rained, it's not been enough. The tendency has been to think that because it's rained, watering isn't necessary. Unfortunately the drizzle hasn't done the job, and many plants in containers have succumbed as result.
Phil J