You didn't give us anything to go on. For someone to help you we need to see the formula with an explanation of what it's suppose to return along with a small sample of your data.
So the following is just a guess.
Did you enter the formula with CTRL-SHIFT-ENTER? If not put the cursor on the cell press the F2 key for edit and then press CTRL-SHIFT-ENTER.
Is the formula inside the IFERROR function. If so remove the IFERROR function to see what error is being returned.