Excel file gone bad

I have an Excel 2000 file that I was using to keep track of
information on advertising. It contains no formulas, the file is all text.

Now it won't open and the reported size is about 10% of the real size.
If I try to open it it says "file format is not valid"
I ziped it (after it went bad) with winzip and when I unzip it Winzip shows
the real size of the file but I still get the error message
when I try to open it.
Can anyone help