You can try IRFANVIEW, GIMP and other viewers to see if other tools will decode it.
Just for better help in the future. You may want to tell a bit more, such as the OS you have available since IRFANVIEW isn't on the Macintosh. If you can post said .JPG on a web site for others to dissect, then that can clear up the issue as others can peek inside the file.
How can I fix jpg files w/ error msg of "can't read file header! Unknown file format!"?