If your start button disappeared, I have the fastest possible fix for you. It works with Windows 7, Windows 8, and Windows 10, and server versions, in addition to versions of Windows you shouldn’t be running anymore.
Hit the Ctrl-Shift-Esc key combination. This pulls up Windows Task Manager. Click File, then Run New Task. Type explorer.exe and click OK. This launches Windows Explorer, which will give you the Start menu back.
This problem happens when Windows Explorer crashes. Typically when it happens, it leaves you without a Start menu and without a desktop shell, and no clear way to launch either. The problem is much rarer than it used to be, but it still happens from time to time.
There are a couple of ways to fix this, but my trick with Ctrl-Shift-Esc is the fastest.