Today I got a error when switching on my Microsoft Windows 10 powered desktop. The error, a critical one said Windows 10 start menu and Cortana could not start, an attempt will be made to fix it after the next login.
The start button in fact was not working and the error message in a popup window was always there, with just one button to sign out. I tried that, logged out and then tried logging in, but again got the same error.
The next thing I did was to use the well known ctrl + alt + del combination and then use the power button icon to restart the system. You can also log off and then press the power icon, select restart, that also works. Now it is important that you select restart and not shut down. When restarting the software started updating, the message said do not power off the system while the update is in process.
After the reboot, and after a small update again, windows 10 was working perfectly fine.
So in case if you get the Windows 10 start menu and Cortana critical error, it probably is some software update that failed mid way, and the best way to most likely fix it is the restart the system, it worked in my case.