When Excel does not calculate a formula, the cause is usually calculation mode, text formatting or an invalid formula reference. Start by checking whether the cell begins with an apostrophe or has been formatted as Text.
Start with the obvious checks
If Excel displays the formula instead of the result, change the cell format to General, press F2 and then Enter. If values are not updating, check Formulas > Calculation Options and switch to Automatic.
What else to check
Also check for circular references, calculation errors in precedent cells and formulas copied into the wrong range. Recalculate the workbook and test the formula with a simple known value.
When to get help
Describe what the formula should do and share the current formula if you have one.