less-lucky case: There is serious damage to the notebook and you will need a salvager program to pull out the good parts of the notebook.
Repairing Corrupted Notebooks Using the AuthorTools Package, for the traditional method.
Another salvager is in the attached notebook. This one usually, but not always, works better than the traditional method above.
If lots of notebooks are having trouble opening, it could be a problem with Mathematica's initialization files. Some can be reset to default contents by holding down the SHIFT key as Mathematica starts. Resetting more of the files is system-dependent; see
Fixing Common Problems by Resetting Mathematica to Its Default Configuration.
Attachment: NotebookSalvager.nb, URL: http://support.wolfram.com/kb/3300,