All the options just kind of disappeared. Various methods I've tried and failed include:
shell:startup
ctrl+alt+del startup menu
Windows Settings>Apps>Startup
/ProgramData/ path to the startup folder (created one to see if it'd populate, it didn't)
/AppData/ path to the startup folder
I'm pretty tapped out on ideas at this point. I can't turn on or turn off any of my startup programs; without this directory it seems like I'm locked out, even though programs are definitely starting (including newly installed ones with "Boot at Startup" enable at install). Anyone ever experience something like this?
You could also try Autoruns from Microsoft Sysinternals, you can find here: Autoruns for Windows - Windows Sysinternals
to see if there are any registry keys related to Start Up apps.
What about hidden files, did you enable the option to see them: View hidden files and folders in Windows
Did you install anything prior to the disappearance of the Start Up menu/files?