Main menu does not show submenus

I just noticed my main menu does not show sub menu's anymore. Tested in different browsers but same result. I don't know for sure if this is a wordpress 4.5.2 issue. Please help. It always worked like a charm and I haven't changed the website in a long time.

Hi!

Could you clarify whether the main menu doesn't show sub menus on the mobile/tablet version of a website or on the desktop one, please.