Sounds like you right-clicked & chose to stop sharing the folder. You can re-add it. See How to Share Folders (click on link).

Or are you talking about the Library window? In that case, go to LW's View menu > Show/Hide & choose Library.

Or are you saying only part of the window disappeared? If yes, then there should be some up & down arrows on the screen. Click on those & all parts of the window will appear. Those arrows have the option of showing part of the window only & filling up the screen or showing both parts.