With the Excel extension is not valid error, try starting the app in Safe Mode, repair the workbook or open the file in another program.

Every now and then, when I try opening an Excel file, I get a message saying that "Excel cannot open the file 'file.xlsx' because the file format or the file extension is not valid.

For those cautious about third-party software, LibreOffice is a free and open-source alternative that supports various file types. If the file opens in LibreOffice, save a duplicate, which will then ...

If Excel is not opening files, you need to adjust your DDE settings or try using other solutions from this guide.

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 you are getting the Data source reference is not valid while trying to create a pivot table in Microsoft Excel, see this post for the fix.