I am using notepad++ V6.4.2. When I close a file, it seems to save defaultly without asking me. How can I configure to get the save/not_save option back?
Notepad++ isn't actually saving the file automatically. If you open the file in another editor you will see that your changes have not been committed to the original file. It is using its own temporary copy of the file to hold the changes that have been made.
However, you can prevent Notepad++ from behaving this way by un-checking Settings->Preferences->Backup->Enable session snapshot and periodic backup.
Edit: As Rob has pointed out, this applies to version 6.6 and later where this feature is turned on by default and doesn't require add-ons.