Hi welcome to GoY, if it hasn't been kept well watered, it could be that, but unfortunately it looks suspiciously like ''box blight'' { Cylindrocladium buxicola } to me, for more information have a look at;
https://www.rhs.org.uk/advice/profile?PID=1005
Derek.

I suspect that lack of water is the culprit. It looks like that it's still in the original pot so it's probably full of roots and would have had to be watered virtually every day which would be the case when it was in the nursery or garden centre. Under the circumstance I would say that it's close of play for the buxus unless you want to chance re-potting and regular watering but could be hit and miss.

Even if it recovers, it may be years before it is back in shape.