Although the symptoms look consistent with dieback produced by Volutella, which is a fungal pathogen, it's impossible to know with certainty based on photos if it's Volutella or boxwood blight (another fungus). Learning the difference is important because boxwoods do not recover from boxwood blight.

So, the first step is to send samples to our OSU Plant and Pest Diagnostic Clinic that include leaves that are discolored but remain attached as well as stem tissue that is likewise discolored but still shows evidence of life. Here is the Clinic website that provides instructions on sending samples.

https://ppdc.osu.edu/home

If they find it's Volutella dieback/blight, management focuses on removing damaged and dead stems, cutting them back until you see clear, white wood in the stem showing no discoloration. Volutella infects stems that have been damaged or stressed in some fashion. So, spraying a fungicide will not provide a cure. Removing the damaged stems allows the boxwoods to fill in the openings.