There is a tool called Advanced Excel Repair. I have used it to repair
many corrupt Excel xls files successfully. Its homepage is http://www.datanumen.com/aer/ Maybe you can try to see if it can
recover the file to the former format.
Ive had instances where the file was saved by auto recover in a temporary doc
folder. If you're in a company, and they have this option, you might want to
check that. Ours are on a whole other drive.