Problem in functions

Hello,

I have problem doing calculation in excel functions, whenever i type a function, i dont get the result of the function but i get the function itself. say i want to add cell B:5 to f:5 so i did =sum(B:5 f:5) but instead of the answer,I get the function displayed as at is i.e =sum(B:5 f:5) and not the result. I am using Excel 2007...Please let me know where is the problem..