excel can no longer recognise format
 

I attempted to utilize Microsoft Briefcase to serve as an ongoing
backup for a folder which contained quite a few excel spreasheets.
After I set this up, I went back in to access one of my spreadsheets,
only to find that Excel would not pull it up, and gave me the "the file
is not in a recognizable format" error message. Further, every file
within the folder that I linked to briefcase is in the same condition.
I have tried every fix I can think of, but I can not seem to retrieve
the data. Does anyone know of a solution?