After a power outage a particularly important file gets the error "Excel cannot open the file.xlsx beacause the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file. The file format has not changed. Is there a safe free program that can uncorrupt a file? I have tried Open and repair, changing the format, and using open office. Also tried using recova but the file was not lost.