The configuration isn't copied to RAM during upgrade, it remains in place on the disk and isn't touched then. Not sure what might have happened, but it wouldn't have only been from the upgrade being interrupted. Not unless whatever happened also corrupted the filesystem.
The upgrade process doesn't touch the disk in that way, it only applies new OS packages in place of the old ones and makes config adjustments afterward. If it gets interrupted in the wrong place, such as during the kernel package update, I could see it failing to boot like you showed. But that wouldn't result in losing the config or other filesystem data in most cases.
It's possible there was some existing filesystem corruption that was exasperated in some way by the upgrade but that's also quite rare. If that was the case, a reinstall should fix it all up, unless there is a hardware issue underneath such as a failing disk. You might want to keep a close eye on it and make regular backups just in case. Turning on ACB can also help there, but you'll still want to retain a local backup periodically, especially using the new option to backup SSH keys, as that will result in a smoother experience with ACB overall.