Why has the menu disappeared on all of my courses?
If you go into a course and everything has disappeared, you have probably accidentally hidden the course menu.
It's far from obvious, but if you hover over the grey bar at the left of the screen, a message appears saying "Show Course Menu". Simply click here and the menu will be restored.
Click where shown
This is a deliberate feature of Blackboard 9 - it allows you to make best use of the space on screen. But it's very disconcerting for anyone who hits that button accidentally - especially as, once you've hidden the menu on one course, it's hidden on all your courses.
The other way in which you can partially hide the menu is by clicking on the arrows as shown below.
This is a feature which has some advantages for staff users, but is probably not particularly useful for students.