A random and very irritating error periodically crops up with Win 10. Sometimes you log in you are greeted with – Critical Error – Start menu and Cortana aren’t working. We’ll try to fix it the next time you sign in. When you log out and back in, it should automatically fix itself.
HOWEVER … it doesn’t always work.
I’ve gone through about 5 separate fixes, including an incredibly complicated one that did bugger all. I did stumble across a fix that seems to work every time. Click on Sign Out -> bottom right click on Shut Down icon. DON’T SHUT DOWN but do a REBOOT. Shut down won’t fix the problem, but a reboot clears it up every time I tried. Before banging your head on the keyboard, give it a try.