My main menu button doesnt take me to my menu

when I press the main menu button(4 white squares-Windows logo)it vibrates but doesn't take me to the main menu yet it works to take me back to the home screen(for example if I go on to Cortana).I am unable to use anything on my phone-Microsoft Lumia 650 as I cant get on my main menu/app page and can only get on Cortana as its the button(magnifying glass pic) next to main menu button.PLEASSSSSEEEEEEEEEE help me.