The two books I have neithor offererend an explanation of how to get back to all thumbnails instead of the last one shown before closing.
Are you saying that if you double click an image in Organizer (so that Organizer now shows single image) and re-launch it you are shown only one image (which was doble clicked last time)? If that is so, then it as as designed that is Organizer remembers your settings from last session. To see all your thumbnails you can click on arrow back button on top left hand corner in PSE10 or earlier (in PSE11 you have 'back' button shown on sort by bar). Please let me know if I missed something.