Press Tab to hide the side panels, Shift-Tab to hide all of the panels, and repeat to bring them back. Alternatively, clicking on the black outer edge strip of each panel controls the individual panel groups. If only a single panel has gone missing, you may have deselected it. Right-click on another panel header to show the context-sensitive menu and put a checkmark against the name of the missing panel.
Article Pages > Where have my panels gone?