File Error since updating to 2007

[KentKHI]

I recently updated to Excel 2007 and ran into a huge problem. One of my worksheets that has hundreds, possibly thousands of formulas in it is popping up an error when I open it that says "File Error: Some data may have been lost" The workbook works fine when I close it, but as soon as I open it, it gives me the error and then deletes hours of hard work. What do I do? The formulas show as =#N/A

I am at my wits end, as I have worked hard on these and do not know if I will have to totally redo them or what? When I got the error, I closed the file without saving changes. I don't know what to do now?

 

[KentKHI]

I have done a slight amount of research and thought it may be helpful to know that I previously was using excel 2003 and had files saved as .xls I am now saving them as .xlsm and in some cases .xltm

Not sure if this would have anything to do with it, but it may????