there are 3 areas on your HD that are used by Calibre
1) the
program files section (needs admin rights ti create)
2) the (standard) user settings (usually hidden). This is where your preferences, current Library name and LOCATION and plugins are kept
3) The Library (the Author and Book folders)
If you did not touch the preferences folder, you probably MOVED

metadata.db
What does the
titlebar of the empty Calibre Library window say
(Mine says MainLib, which is the Library name and the name of the folder that contains
metadata.db)
Is that name match where you found the books (authors folder)?