Boxwood shrubs, when healthy, add to a classic landscape design. When they are diseased and yellow, they become an eyesore. There are several different types of fungi and disease that can attack a boxwood. There is even a term for this sort of degeneration: boxwood decline. This is usually the result of a number of bacterial infections caused by shock due to improper conditions or care.
Examine the leaves to determine the nature and extent of the damage. This will help you determine whether the tree is worth saving. In most cases it will be, so long as at least a third of the bush looks reasonably healthy and alive.
Determine whether the boxwood has received too much or not enough watering. During periods of drought, boxwood decline can set in, whereas not enough water can cause root rot. Both will result in yellow and brown leaves.
Understand that parasites called nematodes cause yellowing of leaves as well. Nematodes cannot be seen by the naked eye.
Look for yellow leaves, which were first orange or bronze, on the tips of twigs. This likely means your bush suffers from stem blight.
Understand that there exists a disorder among boxwoods which turns otherwise healthy, green leaves marginally yellow for which no cause has yet been identified. This disorder does not seem to affect the shrub, however, and is not a cause for concern.