A note on prevention: The most common source of a corrupt MXD file is the thumbnail image. Thumbnail sketches are snapshots of your MXD file that are automatically created every time you save your ArcMap projects. Thumbnail sketches allow you to preview your MXD file in ArcCatalog, and for this they are handy. However, the thumbnail sketch is the element most often corrupted in a broken MXD file. The easiest solution is to turn off the thumbnail image.
Under File > select Map Properties. Uncheck the box ‘Save thumbnail image with map’. Save your project. This must be done for all your MXD files, but remains set for each from one session to the next. Incidentally, this makes saving your project run much faster!