[BetterRed]

@chieflx - how did you do the update from W7 to W10, via Windows Update (using the Get Windows 10 'offer') or from a DVD or ISO file? I'll assume the former.

Are you logged into W10 with the same User Name that you used on W7? Calibre keeps its configuration data (preferences, plugins, customisations etc) -- including library locations -- in the directory %SystemDrive%/Users/User Name/AppData/Roaming/calibre. If you're logged in with a different user name you will be accessing a different set of configuration data.

Do you know where your library folder was before the upgrade - if not, try your Home (%SystemDrive%/Users/<User Name) and Document (%SystemDrive%/Users/<User Name/Documents) folders - it is probably named "Calibre Library".

If you find it then you can use the Library->Switch Create->Use the previously existing library at the new location to 'attach' that library to calibre. If its in 'under' another User Name than the one you're using then you may (probably will) run up against Permission problems - in which case I would ask why aren't you using the same User Name?

BR