Could be box blight. My son just removed two that were about the same age as yours and over this year were completely dead. Some people are replacing with yew but my son can't because they're poisonous to his animals.

GOLDEN EUONYMUS
Its evergreen leaves, trimmed with buttercup yellow, has tiny white flowers in spring not very noticeable though.

The blight hit here and there are a lot of dead buxus around the island. I cut mine down close to the base and a couple have started growing back. There are others that were not affected at all. On the other side of the island, there is a large planting of a type of Aloe that has also been wiped out by some sort of virus, which I hope doesn't make it to my area.

Agree, Golden euonyumus is lovely, and bright all year round. I have two and love them. Long lived, shade or sun, puts up with dry ground, trouble free .Trim- very occasionally if it gets bigger than you want - what's not to like? It can try very occasionally to revert to plain green, but if it tries just cut that stem off.