Whether your file has been corrupted by a virus, hardware problems or software bugs, generally even the smallest problem will prevent you from accessing your data, and the only solution is to rebuild the file structure.

Microsoft Office files use a complex and undocumented compound OLE document format so it isn't worth looking through the file with a hex editor unless you know what you are doing.  There are some services that can recover corrupt files but we prefer to do it ourselves.

When we need to recover corrupted password protected files, we firstly decrypt the file using a key recovery program, then use Cimaware Software recovery tools.

In particular, we use ExcelFix quite regularly.  It is simple to use and delivers good results in repairing files that are corrupted - the files open up easily after using this tool.

Cimaware Software provides ExcelFix, WordFix and AccessFix to repair corrupted files.  OfficeFix combines the functionality of all three packages into one.  You can download the trial versions to see whether the software will recover your file before committing to purchase.