I have entered a formula for adding accross worksheets ie =sum('Jan:Dec'!B2)

I use this as I have a workbook that contains eorksheets for each month and
another sheet for the annual totals. I want to add the values in the monthly
sheets to get the annual totals.

It was working until I changed the names of the worksheets and added an
extra worksheet for Jan (previously had only Feb to Dec). I tryed to delete
the formulas and reenter to include Jan but now it comes up #REF!Error .
does anyone know how i can fix this.

thanks