file format or file extension is not valid...error message
 

Hey Guys, I am receiving the following message on my 2007 Excel

"Excel cannot open the file 'file name' because the file format or file
extension is not valid. Verify that the file has not been corrupted and that
the file extension matches the format of the file.

What happen was I had an .xlsx file saved from 2007 Excel, opened in office
xp excel, downloaded the compabiltity program for office xp excel, edited the
file on office xp excel, saved the file, closed it.

After that, when I try to open the file again, it pops up the above message.
I have tried changing the extension to .xls, still doesn't work.

What can I do now to recover my file? The file is saved on an external hard
drive.