Why can't i insert a new worksheet?

[beaker15]

I have an Excel 2000 spreadsheet that has about 25 worksheets in it, all
quite small. It won't let me add a new worksheet though, nothing happens when
i click the icon for it and when I right-click the tabs at the bottom the
'Insert' option is greyed out. I have mostly been trying to open it in Excel
2007 but I also tried on a pc using Excel 2000 and get the same result. In
2007, i have checked its in a trusted location and i can make other changes
to the spreadsheet.
any ideas?

 

[Gary''s Student]

1. make sure there are no protections in place that prevent adding a worksheet
2. open the workbook in Office Safe Mode and see if that helps.