Andy has updated his mom's computer with Windows 10 Home. He's got her Defender AntiVirus all updated. But now, her start menu won't come up and the Windows key doesn't work. Should he look at the registry? Leo says NO! Trust the solutions at Microsoft.com. Leo says that any user who isn't sure what they are doing can mess things up in the course of learning to use their system. The reality is, Windows is too powerful for your mother or someone who isn't a sophisticated user. Leo recommends replacing that Windows 10 system with a Chromebook. Meantime, try booting up in safe mode and see if the start menu reappears. If so, then that will point to a conflict with a driver or some installed software.
Another way to check is to create an unused Windows account you can compare it to. And check to see if the OS hasn't been put into tablet mode. It's VERY easy to do. Here's a few other suggestions - https://support.microsoft.com/en-ca/help/12385/windows-10-fix-problems-with-start-menu