I've been playing several modpacks, just searching for some that I'd enjoy. A lot of them went to a grey screen and switched through grey, blue and green when I moved my cursor. I could press ESC and it would go to the menu, I could go to the main menu as well. It just..remained colourful.
It didn't crash or anything, it just stayed that way.
It was an issue with several modpacks, but I'll heresthe one I used with this specific video: