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.

2] Save the Excel workbook file on the Local Hard Drive This is another workaround that you must surely try before following any complicated fix. Try saving the Excel workbook file to the local ...

File-security limits can curtail your ability to open or save Microsoft Excel files.

If you have a spreadsheet from Works versions 6 and later, you can open it directly in Excel. If you used an even older version, you can re-save the file as an Excel formatted file from within Works.