Every time you make a pixel change in a photo, Photoshop Elements saves the state. You can jump to any of the last 50 (default and configurable) states by selecting the state in the Undo History panel and start working from it. You don’t need to save a change in order for the change to appear in the history.
If you can’t find the Undo History panel in your workspace, open it from the Window menu.
For more information about the Undo History panel and a set of guidelines, see this article in Photoshop Elements Help.