Notepad++ has two ways of backing up the data: periodic backup and "backup on save". Both options can be accessed using Settings > Preferences > Backup. By default, only the first option is configured and you can access the files using
%AppData%/Notepad++/backup. The files that appear there will be the ones that are currently opened in Notepad++.
If the second option wasn't activated, then it's impossible to recover the files. Locate the Backup and Save section then choose Simple backup and assign a custom directory. You will have the possibility of recovering the files from now on.