Fixing a severely damaged image might involve editing over a period of hours or during multiple editing sessions to get good results. In the process, you will create a number of layers and you might want to experiment with different techniques to see which ones work best. Some basic workflow planning will make the editing process go more smoothly.
The first and most obvious caveat, but one that bears emphasizing, is that you should never work on your original image. Make a duplicate of the original image to work on and keep the original in a safe place. If you're using a scan, use a high-resolution scan—it's easy to reduce the size later on and very difficult to get satisfactory results increasing it. As you work, save the image and make regular duplicates at strategic points in the editing process. If you make a mistake later on, you can always return to the previous version of the image.
Fixing Errors with History
Photoshop history is lost when the file is saved and closed. So, although you can use it to roll back changes that you made to the image in the current editing session, you can't do it after the file is closed. To roll back changes, display the History palette by choosing Window > History and then step back up the sequence of steps from the last one upward to locate the place from which you want to continue.
By default, Photoshop history is limited to a small number of steps that can be undone. When you're applying a lot of fixes to an image, you need to enable a bigger undo history so that you can roll back changes during this session. Choose Edit > Preferences > General and set the history state to a higher number of levels, given the size of your computer memory and the size of the image you're working on.