Unresolved is one thing. When there are things left unsaid and a loved one dies. The tension is great. But even after the death, you can re-remember it. You can re-parent yourself with the bits and pieces - like a patchwork quilt, we make patterns that heal and have a beauty all their own. Perhaps not of our choosing, but from what is.
But what if you have little or none of that? What if the parent did not care enough or could not for some systemic malady, hang around. The child, the adult child will constantly have that parallel universal question hanging over them, like a cartoon character's dialog bubble.
It is unrequited parenting. It is deeper to heal. The void is fjord sized.
It is the saddest story I have ever heard.