It’s always hard to troubleshoot problems like this, not knowing what another user’s computer environment is. However, if the remember current session for next launch is checked, as @Claudia-Frank suggested, find where the configuration files are saved. There are two helpful articles in the Wiki on this: Notepad++ Install Folder and Where are the files?
Once you find these, write down the path. Then I’d try copying a random text file to that folder, or perhaps creating a uniquely-named folder there. Then the next time you open Notepad++ and it doesn’t open the last session, check that folder and see if the files there are the same ones that were there before. If that text file and/or folder are also missing, you may be on a system that wipes the folder when some criteria is met.
As a workaround, you can manually save your session (File -> Save Session) to a folder you know will retain its files, then open the session file when you open Notepad++. You can even include that on the command line in your shortcut to Notepad++ if you want.
