Buxus gone yellow - can it be revived?

[Titsywoo]

I have two pyramid buxus semperviens trees in pots - have had them about 2 years but this winter one of them has gone yellow on the outside and the middle is brown. I noticed that it wasn't draining a couple of months ago so am worried I have drowned it! I made more holes so that isn't an issue now. I have trimmed it back and am hoping it will come back to life this summer but any tips on what I can do or is it a lost cause?

[Ferguson]

These links may help:

www.buxuscare.com/en/potted-buxus

www.bellamont-topiary.co.uk/growing-guidance

www.crowntopiary.co.uk/topiary-care-guide-box-and-varieties/