Cannot save Excel file

Hi, I am trying to save am Excel 2007 workbook, but get the message "Document
not saved" each time.

If I try "Save As", it does nothing, so there is no opportunity to try a
different location or format.

I tried copying and pasting part into a new workbook, but it ignores Save
and if I try to close the new workbook, it asks "Do you want to save the
changes" repeatedly, even when I press Yes.

So I am stuck and cannot close Excel.
Any suggestions?

Thanks