My Excel columns somehow changed from A, B, C .... to 1, 2, 3...

The assignment/reference of cells A1 is R1C1, A2 is R1C2, B1 is R2C1, B2 is R2C2, etc... and my formulas are now unidentifiable. May have been my cat stepping across my keyboard? How do I reset my grid columns with letters and rows with number as commonly used.

Thanks for you help.