Cortana is off by default. If you REALLY want to use it, just enable it.
http://microsoft-news.com/how-to-enable-cortana-on-windows-10/
As far as the start menu, what EXACTLY do you mean by "stopped working"? Do you mean there is no start menu, or that it's appearance changed, or that there is trouble with using it?
I'd suggest using Classic Shell if you want the Windows 7 style start menu with Windows 8.1 or 10.
http://www.classicshell.net/