Depending on the reason why the Start button has disappeared, options include restarting explorer.exe, clicking and dragging the line at the bottom of the screen, changing the presentation mode or disabling the auto-hide feature. Alternatively, updating Windows to a higher version can bring it back.
The Start button is a feature of explorer.exe. As such, it disappears when explorer.exe crashes or malfunctions. To restart explorer.exe, hold down the Windows key on your keyboard, and press R. Type in "explorer.exe," and hit Enter.
It is also possible that the entire taskbar has been minimized by accident. If this is the case, there is a thin line at the bottom of the screen. Clicking and dragging it up brings back the Start button. Alternatively, the auto-hide feature might have been enabled. Windows can hide the entire taskbar, including the Start button, by sliding it down the screen. If this is the case, the taskbar and the Start button slide back up when you move the cursor to the bottom of the screen.
Likewise, if Windows is in presentation mode, changing the projection back to PC Screen Only can bring back the Start button. To do so, hold down the Windows key, and press P. Then, click PC Screen Only.
If the Start button disappeared after the update to Windows 8, it is entirely normal. Windows 8 replaces the Start menu with the Start screen and removes the button from the taskbar. Updating to Windows 8.1 restores to Start button, but the button then opens up the Start screen instead of the traditional Start menu. Updating to Windows 10 restores both the button and the traditional menu.