This is a common query that comes up, when an Excel option has been tweaked or you’re using someone else’s computer and the column headers are set as 1, 2, 3 instead of A, B, C. This doesn’t do any harm, but does make writing formulas a bit more confusing.
Assuming you want to change back to the regular style, you just need to adjust the setting back in your Options. Go to File – Options and open the Formulas menu. In the Working with formulas section there is an option for ‘R1C1 reference style’ (meaning row 1 column 1). Untick this and problem solved.