WordPress is designed to “autosave” your content every 60 seconds by default. WordPress stores autosaves in your browser’s local storage.
With luck, when you go to the page you made changes to, you should get a notification at the top of the screen that reads, “The backup of this post in your browser is different from the version below. Restore the backup.”
If you don’t see that notification, you may also have luck looking in your Revisions. You’ll find this in your Publish box, and will allow you to browse through all the saved versions of the post in your site’s database.
All that said, I recommend saving your work often! I also recommend using the plugin PublishPress Revisions, which lets you save your work without making your changes public yet, so you can save drafts of posts that have already been published.