Content Type "Article" is not showing when trying to add, but it is showing in Structure-> Content Type. Please help me I don't know how i lost Article content type when adding a content. It is not visible for me. 

Please find the two screen shots attached.
Try re-enabling the content types in the navigation menu.

Go to administration->structure->menus->navigation and you should see your article content type disabled in the menu. Just re-enable them and it should all work fine