unable to open an excel file

I am un able to open an excel file that I was able to open yesterday. When I try, I get a dialog box stating the file format or file extension is not valid, and to verify the file has not been corrupted and that the file extension matches the file format. I have tried varies repair and recovery techniques as recommended on the excel help, but no success. The file is located on a thumb drive if that makes a difference. other files on that drive open without issue. Any thought on repair or recovery? even partial would be nice.