A little more Harsh fix. You will lose user SETTINGS and plugins.
Preferences: Miscellaneous: Open Calibre Configuration (click the button) LEAVE EXPLORER OPEN
Exit Calibre
Now delete the contents (or move it to some place safe)
Start Calibre. If you don't use the default Library name/Location, you may need to use
Create/Switch: USE EXISTING: <point it to your Library folder>