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.

If your Excel files are opening in Notepad on Windows PC, here's how you can reset the settings to make .xls, .xlsx, etc files open in Excel.

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.