Sometimes when you're editing a photo, you can get a little bit carried away. It happens to me when I start rotating, cropping and then applying filters.
Your iPhone keeps the original version of the photo, which you can revert to, if necessary.
You can easily see the original version of the photo, by long pressing anywhere on the screen.
As you're pressing, you'll see the original version of your photo. When you release, it will revert back to your edited photo.