I’d suggest reposting in the Perennial Forum since Brunnera doesn’t fit under any of the headings chosen. That’s where people with experience of growing it are most likely to see your question. Also provide your geographical location, describe the growing conditions and say how you’ve cared for it. Best of all are photographs of the situation.

Perennial Forum is here: https://www.gardenweb.com/discussions/peren

I planted two Jack Frost Brunnera 10 years ago. They have grown like weeds. I have dug out huge plants for a relative. There has to be some other reason for the sudden drastic decline.