Thank you for the question. Pachysandra grow best in full or part shade and cool, moist conditions. I see green leaves strewn on the ground as well as many dried up old leaves, but can't see more detail than that. Diagnosing plant problems comes down to figuring out if the problem is insect or disease, or something else like too little or too much water, wildlife grazing, mechanical damage from yard and garden tools, and so on. The most common insect and disease problems that affect pachysandra are euonymus scale and volutella blight. Look very closely at the leaves on the ground and the stems of the plant for signs of the problem. Check your other pachysandra plants that might just be starting to show symptoms.
Read this Penn State Extension publication on Volutella blight, a fungal infection that causes pachysandra leaves to develop brown or tan lesions, dry up and fall off: http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/pachysandra-disease
Euonymus scale infestation causes leaf yellowing, plant stunting, and possible death of stems or the entire plant. The scale live under a waxy dome and have piercing sucking mouth parts that such the juices out of the plant. You have to look closely at the plants to see if you find this scale as it can go unnoticed for a long time until you start to see the damage. Read about it, along with control measures in this University of Illinois Extension publication: https://web.extension.illinois.edu/cfiv/homeowners/020803.html
Here is a good publication on diseases of groundcovers. There is a nice section toward the bottom listing steps to take to promote the health of your plants.
Thank you for contacting Extension.