If you are able to open the file in Excel, you can try to save the file out into different formats and then re-open them in Excel and try to save them back as Excel workbooks.
Click on File and then Save As. Choose SYLK (Symbolic Link) from the Save as type list and click Save. Then close the workbook.
Next, click File and Open again, but this time choose the SYLK file. You may have to choose All Files from the Files of type list to...