What is killing off pachysandra? It was great for aprx. 12 years?

They could be over-crowding themselves. I would thin them out.

As well as Jane's advice, have you checked the undersides of the leaves for any insect damage? Pachysandra is a lovely ground cover; I hope you soon find out the cause. Have you researched any info on Google? ... or asked your question at your local nursery?

Mine has been getting thin and leggy and it's been about 15 years. I've been thinking that I need to dig it all out and restart it. I will follow this for more input.

Pachysandra even though is a ground cover is similar to evergreens. They do eventually use up their nutrients. That doesn't mean you dig up the soil, just add amendments (composts) Straight fertilizer would shock them, possibly kill them.