In Blackboard, the menu on the left-side of the screen can sometimes auto-collapse, leading some users to believe that the menu is not displaying properly. This is a feature that is designed to save space on mobile devices but can become confusing when using a traditional screen.
To open the menu again there is a small clickable area to expand it.
If this persists, the browser may be zoomed in enough that this behavior is triggered more frequently. Try zooming out by holding down Ctrl and pressing the minus symbol (-). If you zoom out too far, you can zoom back in by holding down Ctrl and pressing the plus symbol (+).