A random and very irritating error periodicaly 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 seperate fixes, including an incredibly complated 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.