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 you see The file couldn't open in protected view error while opening Microsoft Excel, the fixes listed in this post will help you.

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

How to Open an XLS File in Excel. XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save ...