I don't know how I have managed this, but my columns in every workbook are now numbered 1, 2, 3, etc. instead of letters A, B, C etc.

If I put a formula in any cell, it no longer shows which celss the forula is referring to, For example instead of

=A1

to make the cell read whatever is in A1, I get

=RC(-1)

How do I fix this?