Calibre will create a new (default) Library ONLY when it cannot find the:
1) last one used (uses preferences)
2) the default Library folder does not exist
Common reasons for 'lost libraries'
1) Library was on a network share that disconnected
2) Library was on a removable DRIVE and the letter changed
3) The user stored the (real) library on the desktop that got 'cleaned up'
4a) Your Antivirus declared the Library folder 'infected' and placed it into the 'vault'
4b) Automatic cleanup of old files and folders
5) Impending doom: Hard drive is failing (lost sectors). The Windows: event viewer log should have some red flagged entries