I had that problem this past week. Support suggested a few things but nothing worked. Power down - wait 15sec - restart hub and repair z network. Go to graph.api.smartthings site to disable and enable z network (under hubs/utilities). Na dah, and I couldn’t even add back any z-wave device I removed.
Yet after a weird hub offline - back online warning yesterday, ST started to see z-wave devices again. (well, only one which I tried to add back) I was able to add back the rest too, although it was still clunky and I had to try multiple times to get them added.
I don’t think it was individual cases. It may be helpful to list the z-devices of the people who suffered the outage. Perhaps there was a culprit that caused the problem.
aeon labs multisensor (gen1)
GE outdoor switch 45604
GE plugin switch 12719
USNAP z-wave module for radio thermostat
Evolve LSM-15 wall switch