If your library is in a non-standard location another possibility is that the preference file that records which .itl file to open has been reset or corrupted. This may cause iTunes to open or create a library at the standard path of <User's Music>\iTunes. In this event, press and hold down shift/option as you launch iTunes to reconnect to the correct library file.
In a similar vein it is possible that a corrupt preference file may cause iTunes to fail at launch with one of these errors:
- The file iTunes Library.itl cannot be read because it was created by a newer version of iTunes.
- The folder “iTunes” is on a locked disk or you do not have write permissions for this folder.
- The iTunes Library.itl cannot be found or created. The default location for this file is in the 'iTunes' folder in the 'Music' folder.
It may be worth trying the shift-start-iTunes method (option on a Mac) to actively select the .itl file that should contain your library. If you still get the error message try Repair security permissions for iTunes for Windows and/or check for interference from anti-virus software.
For more info, see:
Empty/corrupt iTunes library after upgrad… - Apple Community