When Password Protected - The File Becomes Corrupted and I Can Not Open It

Question

I have password protected two files now in Excel 2007.  After opening and then saving a few times, the file becomes corrupted and I cannot open.  The following error message appears  "Excel cannot open the file"XXXX.xlsx" because
the file format or file extenson is not valid.  Verify that the file has not been corrupted and that the file extension matches the format of the file.
The files that I created contains sensitive information and I really need to get to them.l

Answer 1

You can try to open these files with 'Open and Repair' .
1.Start Excel.
2.click the Microsoft Office Button, and then click Open.
3.In the Open dialog box, click to select the file that you want to open.
4.Click the down arrow on the Open button, and then click Open and Repair.
Also this issue could be  caused by malware on the affected machine
Please see:http://blogs.technet.com/b/the_microsoft_excel_support_team_blog/archive/2013/09/07/quot-cannot-open-the-file-because-the-file-format-or-extension-is-invalid-quot-opening-office-files.aspx
Some one works out this issue with remove the xlsx entention,give it a try.
1. rename the file and remove the .xlsx or .xls extention etc e.g. if my file is called test.xlsx I remove the '.xlsx' - you will get a warning message saying this could make the document unstable - accept it
2. the document icon should now have turned white (the document won't look like an excel document anymore)
3. open a blank excel document by hitting start -> all programs -> Microsoft office -> excel (this may be different if your not on windows 7)
4. in your blank excel document hit file -> open
5. navigate to the document you renamed above and select it, now click open