Have the number in the range been entered as text, or converted to text, or
imported as text?
If you're working in Excel 2002 and up you can use the smart tag to convert
the text values to numeric ones, if not you can do the following (after
making a backup copy of the workbook):
Insert a helper column to the right of your numbers
In the first cell enter the formula =VALUE(A1), substituting your first cell
address for A1
Copy down the helpler column to the last value that needs to be converted
Select the all the formulas in the helper column and press Ctrl + C to copy
then
Select the first cell in the text value column and click EDIT in the menu
Select PASTE SPECIAL and click the VALUES check box
Delete the helper column