The grey cube disappeared

I used to be able to choose File … New and start over with the grey cube but now I must have somehow saved over it with my own yellow modified cube that I made as a test object.

I can’t get back to the basic cube anymore since when I choose New, I get my yellow modified one instead.

How can I get back to the grey default cube?

your modified cube then Shift+A / add cube, then Ctrl+U to save as your default scene
File / Load Factory Settings, Ctrl+U to save as default