First reinstall Windows 10. The modern term is refresh or reset. http://www.tenforums.com/tutorials/4090-refresh-windows-10-a.html is just one of the many sites telling you about it.
If that doesn't help, it might be some hardware issue.
Critical Error (message)
Start menu and Cortana aren't working. We'll try to fix it the next time you sign in.
Sign out now
(Tried but it returns each time)