How do I change the labels of my columns from numbers to letters?

[Anna-Maria]

Up until yesterday, the labels at the top of my spreadsheets were letters
(A,B,C...). Today they are numbers (1,2,3...). How do I change them back,
and what might have happened to cause this?

Anna-Maria

[davesexcel]

You are probably in R1C1 reference style
goto
tools=options=general and uncheck it