My Calibre restore went without a hitch. Everything is back to normal again.
I had thought Amazon Support would be the best place to go for advice about what to do, but it turns out, there are far more folk here with knowledge of the problem (and its cause) than at Amazon.
Yes, I did disconnect my Kindle without ejecting it first. As a Calibre user, I use Calibre's Eject mechanism, which is their equivalent of the Windows 'Safely Remove Hardware' tool.
Just the once I let my guard down, and it bit me!