Well, this is not a big issue or error in Excel; this is the common issue that appears in many cases while working on Excel workbook.
If column letters and row numbers are missing then go to View > click on “Headings”. However sometimes the row and column heading are gone and there is not written A, B, C and so on the above columns. Even you can identify the Rows 1, 2, 3... is also missing.
And to show or hide the row and column numbers and letters simply go to the View Ribbon > tick mark at Formula Bar.
This is it.