Thanks for your reply Kovid...
Thank goodness for belt and braces!
My Calibre Library now resides on a server. In a desperate attempt to recover, I just did a hard drive search for a Calibre Library, on my workstation, which was Calibre's original home. Lo, I discovered a backup I had set up for the folder which contains the server copy. I set it up so long ago, I had forgotten I had it. It has been ticking over every night, without any intervention from me.
Calibre is back intact!
Now, all I have to do is discover what went wrong with my Windows Home Server backup.