If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

Close all other open programs Give the workbook a different name Disable Excel add-ins Use the Share feature in Excel Update Office Repair Office. 1] Close all other open programs The first thing ...

If Excel fails to repair the workbook when you select the "Repair" option, open the file again using "Open and Repair," but this time select "Extract Data" instead.