If you reset your preferences without noting your current catalog's name and location, it can appear that your catalog has disappeared when you restart Lightroom. Before you reset your preferences, refer to Recover catalog and images after resetting preferences, which contains steps to avoid this confusion.
(Mac OS): The user Library file is hidden by default on Mac OS X 10.7 and later. For temporary access to the user Library file, press Option and choose Go > Library in the Finder.
(Windows): The AppData folder is hidden by default. To view it, choose Start > Control Panel > Appearance and Personalization > Folder Options. In the Advanced area of the View tab, make sure that Show Hidden Files and Folders is selected. Then click OK.
For more information on important Lightroom files, see the following links:
- Preference file and other file locations | Lightroom CC and Lightroom 6
- Preference file and other file locations | Lightroom 5