When making edits to a page, it is important to save your work often.
Discard Check Out
If you were working on editing a page but have decided that you don't want those changes, you can revert back to what you had when you started the edit. This is done by selecting "Discard Check Out" under the Page>Check-in button. When the message box pops up, click on the "OK" button. This will reset the file back to the way it was before you checked it out and started editing it.
Revert Back to an Older Version
If you have already checked in and/or published the file, you can still revert back to a previous version of the file (or page). This is done by selecting the "Page History" button.
All the versions of the file, with their respective check-in date, will be listed in the left column. When a version is selected, the main part of the page will show all the changes made between that version and the current version. If this selected version is what you want to go back to, the select "Restore this version"
Editing a page involves checking out the file so that you are the only person who can edit this file. This "lock" on the file does not get removed until you "Check-in" the file or "Publish" the file.
The process of publishing a file makes it so that everyone can see the changes you've made. Until you do this, the changes can only be seen by you. Publishing a file is very simple and you just need to select the "Publish" button. This actually does both the "check-in" as well as the "publish". When you are finished editing, don't forget to publish it!
After you select the "Publish" button, the box below will pop up. No comments need to be added. Click on "Continue" button to complete publishing the file.