The Ribbon, menus, icons, settings and many of the items that are present when you start Word up are embedded in the normal template. Sometimes this template may experience corruption. If it becomes corrupt the only cure is to remove it and allow Word to automatically create another.

You can try removing the template and see if Word creating a new one cures it. The normal template is called "normal.dotm" and here is where you can find it:

/Users/your user name/Library/Application Support/Microsoft/Office/User Templates/normal.dotm

Shut Word and any other Office application down and then move normal.dotm to your desktop or delete it but don't empty the trash. Word will create another when it's restarted.