The panels can be set to be open or closed or Auto Open and Close in each Module.
So, you have the Library Module set to have your left panel open, but your Develop Module set to have it closed.
Click on the small arrow in the middle on the left side, and the panel will appear. Right Click on it to set options on how your want it to appear. Use F7 to show the left panel, F8 to show the right, F6, the film strip, F5. the Module picker. Use Tab to hide the left and right panels, and Shift-Tab to hide all panels.
In each Module you can use Shift-? to show many, but not all, shortcuts for the Module.