After installing Windows 10 Fall Creators Update the start menu stopped working occasionally. It didn’t open anymore and window switch (ALT+TAB) didn’t work either until restart.
Here are the steps for the FIX:
1) open Windows Settings App
2) click on ‘Accounts’ Tile
3) click on ‘Sign-In Options’ in the left menu
4) scroll down to the ‘Privacy’ section
5) *disable* ‘use my sign-in info to automatically finish setting up my device after an update or restart’