Pc start button disappeared

Move mouse pointer down to bottom of screen and see if it appears.

If it does, click the start button, Control Panel, Taskbar and Start Menu

Uncheck "Lock the taskbar" and "Auto-hide the taskbar"

If moving the mouse pointer to the bottom of the screen does not make the taskbar appear, press the "Windows Key" on your keyboard and go to Control Panel as described above.

If you have no Windows Logo key, press Control and Escape at the same time. (Ctrl and Esc)