Here's an update in case anyone reading this has any ideas, or a similar problem.
First, I wanted to check that all my hardware still worked, so I cloned a working Windows 10 boot disk (using AOMEI backupper) and attempted to boot my Hackintosh from that disk.
Here's where it get strange... specifying that volumes UEFI as boot failed to load ... but specifying the volume (and I'm sure it was this volume because it's the only one of that size) successfully booted an old backup Clover El Capitan installation.
So the good news is, my computer is working.
From that boot I can see in Disk Utility that *neither* of my previously working High Sierra boot SSDs will mount. The drives show up but when I try to mount the disk volumes nothing happens.
These two SSDs were my primary installation and a backup clone I make regularly using Super Duper.
It seems very unlikely that both drives would get corrupted at the same moment.
Unless that's a clue to what happened?
Meantime I'm using dd in terminal to make a backup of one of those so I have all my apps and preferences. And if it comes down to it I'll do a clean install and restore the whole thing piece by piece.
But that's gonna be a lot of trouble. If anyone has any clue what's going on, or if there's a simple fix I'm missing here, please let me know.
Thanks for reading!