Actually this has been a part of the site navigation since we updated the main page of the site and the header navigation and this will display as a button when the viewing window is too small for the display with options along the top of the page. This is a part of increasing the mobile friendliness of the site so there is an effective way to navigate on a mobile device.
You should be able to see the navigation option along the top of the page if you have your browser window maximized on your monitor (depending on the resolution of your monitor).
If you are unable to see this appropriately in a maximized window on your monitor and were able to previously, you may need to ensure that you have not zoomed in on the page. If you continue to run into trouble, please feel free to file a support case from under the Help tab of the site letting us know the details of what you are experiencing as well as the browser and operating system you use and we'll be happy to assist you further.