Add Books from an INTACT library is not the way to go if all you want is to get it back on the Quick Switch list.
It appears that your Calibre preferences became corrupt (where these kinds of things are saved)
All you need to do is reattach Calibre to the lost Libraries.
On the Library Icon: Right click: Switch/create: Use Existing: <Navigate to the Lost Library folder that has metadata.db>
Repeat as needed for additional libraries
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