Please ensure the .xls file is not opened in other workbooks, add-ins, or apps. After coming to that conclusion, simply attempt to open the file again in Excel to see if the problem no longer exists.

If Microsoft Excel fails to recognize a file in the XLSX or XLS format, the data contained in the file could be corrupted. When you open a corrupted workbook in Excel, the program should ...

Microsoft Excel has an import wizard, but if you structure the text correctly, you can bypass the wizard altogether.