You can set preferences to change the Lightroom user interface.
If you reset your preferences without noting your current catalog name and location, it can appear that your catalog has disappeared when you restart Lightroom.
Before you reset your preferences, refer to this technote, which contains steps to avoid this confusion.
Open the Preferences dialog box
Restore preferences to default settings
See Preference and other file locations in Lightroom 5 for more information on important Lightroom files.