If you find that your navigation bar has disappeared, then do look at the device that you are logged in on.
If you are on a mobile, or a tablet, or have a small screen then the chances are the responsive nature of the system will have delivered you the mobile navigation button.
Below is the standard navigation bar on the left on typical modern laptops/desktops:
Below we show the mobile header. Please take note of the button in the top right hand corner:
Once the button is clicked/pressed, you will notice that the menu will expand, showing all of the options that you would have seen before: