If the workbook name ends with ".xls" instead of ".xlsm", it is likely the file was copied or e-mailed and the last letter of the extension left off. Try copying the file (in Windows, without opening in Excel) and change the name of the copy to "[filename].xlsm".
If you can then open the new workbook through MasterMine, you're good to go. (At that point, delete or archive the .xls version to avoid future confusion.)
Also here are some general tips for when you think your MasterMine workbook may have become corrupted:
1. Copy the file first (in windows) before trying to open it in Excel. That way you can be sure you don't lose anything and have a backup.
2. JUST BEFORE opening it in Excel, run MasterMine and click "Open" to initiate the "File Open" dialog so that the machine registers the MasterMine license -- then just close the open dialog. Then switch to Excel and open your file in that dialog.
3. Another possible issue is that you are opening the workbook using an outdated release of Excel, possibly one that has not been updated with current service releases. Excel 2007 and up are all fine, but make sure your Excel has the latest service releases available.