This groundcover commonly gets a disease when it ages, which is controlled best not with spraying anything, but with management of the bed. Response is to renovate the bed by raking out leaves and dead vines (or mowing it down), and reducing the mass of the plants so that air circulation is improved and leaves don't stay wet and the bed doesn't hold moisture for long periods of time, which encourages disease. Mowing sounds extreme, but pachysandra has extensive roots with a great deal of stored energy and it can bounce back with new growth.
Here is a more extensive explanation on our website, with solutions:http://extension.umd.edu/hgic/volutella-dieback-pachysandra-groundcover
If you look at the photos of volutella and see no spotting or stem disease, please send us photos, including close-ups, of the problem. Show dying plants, not completely dead ones.