GIMP is an Open Source program for manipulating images. See the About window:
When starting the program, several windows are opened including (usually) a Layers Dialog box which gives you a way of manipulating the layers of a composition. When closing, however, the temptation is to close all the windows visible.
All you need to do is to close the main window (the one with the File menu).
By closing the Layers Dialog before the main window, next time GIMP opens it will not display that dialog.
In previous editions, there was a Dialog entry under File to help you to recover the missing window. However, in the current (2.6.7) version, you must go to Windows | Dockable Dialogs | Layers to restore sanity.