The problem of "missing panels" has become increasingly prevalent, so I thought it worth posting a quick reminder here in the Tips, Tricks & Starter Kit thread that this "problem" is in fact merely an option within Lightroom's UI (all versions).
In all of the modules, both the left and right-hand panels can be configured by the user to hide or show almost all of the individual sections. This is achieved by simply right/ctrl-clicking on the header of any already-showing section (or empty space) in a specific panel, whereupon the option to select/deselect all the sections of that panel will be presented. In this screenshot of the right-hand panel in the Develop Module, I have previously hidden the Tone Curve section. By right-clicking on any of the other section headers, the ensuing context menu allows me to restore the Tone Curve section by clicking on it (or Show All), and I can also use the same method to hide any of the other sections:
This same method of showing/hiding individual sections is common to all modules. The only exceptions to this are the Navigator (Library, Develop, Map), Preview (Book, Slideshow, Print and Web), and the Histogram in Library and Develop. None of these can be hidden (although they can of course be collapsed).
Another feature that was introduced in Lightroom 4 is the ability to hide/show all the individual modules. This is again controlled via the right/ctrl-click function, this time doing it anywhere in the right-hand side of the module-picker bar, see this screenshot: