In the login bar -- usually towards the top of the page with your login name, a log out button, and a link to your preferences -- is a link labeled "undo".  It takes you to a screen showing all the changes you've recently made to your website.  Unfortunately, the explanation at the top and the descriptions of the changes themselves is fairly wonky -- this is a thinly veiled window into some of the underpinnings of Plone.

The changes that Plone saves are on an "action" level, where an action is something like a user hitting the save button (saving a number of changes to a single page), cutting and pasting a folder, creating a document, or elevating the state of a page from "visible" to "published".

To undo something you've done, you need to find it in the listing (which may not be easy, given how cryptic and extensive the list is), check the box to its left, and click undo at the bottom of the page.

Note that there are limits to when can be undone -- if you create a page, and then later move it, you cannot undo its creation, since subequent actions (your having moved it) depend on its having been created.

Plone has the potential to remember everything that is done to your site ever.  However, you can imagine that that generates a lot of overhead (in the form of used disk space), so we periodically go into the management interface and tell Plone to forget everything that's happened that's more than two weeks old.  So don't count on being able to undo changes you made last month!