Formulas disappear from cell and keeps values only

[gmgree]

Help! I enter a SUM formula into a cell to calculate a range of cells and once I return to the cell it has a value. The formula disappears. I input the formulas correctly =sum(b4:b354) but when I return to the cell the formula bar and cell both indicate it as 54 not the formula. I don't even get an error option (little arrow in the top left corner of the cell).