Is there a skinny little bar on the right side of the window with an arrow pointing toward the center on it (the arrow is small and in the vertical center of the bar). If so, then you accidentally collapsed that side bar. Just click the little arrow and it will expand back into place. If you didn't collapse it, it's also possible that you accidentally dragged the width of the sidebar to be hidden. Ti fix that, move your mouse slowly toward the skinny bar until you see the cursor change. You should then be able to drag the skinny bar toward the center. This will expose your sidebar.