Why Do Columns Show Numbers in Microsoft Excel?
By default, Excel uses the A1 reference style, which refers to columns as letters, and refers to rows as numbers. These letters and numbers are called row and column headings. However, Excel can be configured to display column labels as numbers instead of letters.

Some people may prefer this method but it can also be confusing, especially if it is enabled inadvertently.

Columns showing numbers in Microsoft Excel

Change method back to normal

To change from this method back to normal go to File > Options and then Formulas and un-tick the R1C1 reference style box.

Formulas option in Excel