Help Excal 2003 error on 1 file

[Renee]

I am trying to open a file I was in last week and then I
do it says "File Format is Not Valid". It is a .xls
extension and worked fine last week. Now another girl at
work was init yesterday and saved it however this has
never been a problem before and she cannot remember doing
anything abnormal. Please help I really need this data. I
don't know how to get it back. I even tried making a copy
and renaming it and I still get the same error. Any
suggestions

Thanks in Advance