I apologise if this is a bit of a stupid question, I have had some training
in the past on Excel but not for some years now and so im a bit rusty. At
work ive been working on an Excel file containing statistics, involving a few
formulas but ever since I re-opened the file recently, something has confused
and annoyed me.

What it is, is where usually the rows are all numbered and the columns are
labelled alphabetically (so for instance B3 is the third cell down and the
second cell along from the left), for some reason now both the columns and
the rows are numbered numerically... this wouldn't be so much of a problem if
it weren't for the fact that it confuses me when I try and enter formuas, I
know you can click and drag the mouse over the cells and it automatically
enters the right cell code but it just doesn't seem to make sense, where one
cell far down might be cell 5 (column no.) 145 (row no.) but if I click on
that for a formula, it labels it as (where I type SUM=( to indicate the start
of a calculation/formula) '=SUM(R[-304]C[1]'

This is probably an easy fix but I would love it if I could change/reset it
back so that columns have the usual alphabetical headings... I just don't
understand the above at all, where does the R and the C and this 304 come
from, so it bamboozles me! plus ive no idea why its changed the column
numbering, unless I accidentally hit something by mistake(?), it was right
when I originally started the file im pretty sure... I also wondered if it
was a setting on this one particular computer and managed to try accessing
the file on Excel on another desk computer but that had exactly the same
column heading format, so I guess its not that. It stays the same confusing
format when I try to create a new document too... confusing...

Any help is much appreciated, many thanks in advance.