Excel Column Headers changed to numbers

For some reason, the column headers in my excel 2000 changed from the standard alpha designators to numeric designators. Now I have numbers as both column & row indicators. Does any one know how or why this happened & how I undo it? (There are no macros or anything like that.) As far as I know it happened after I created an excel workbook from a quickbooks report. (something I’ve done multiple times before without this happening.) Now all excel sheets open like that. When I tried to open my original document on another machine with the “normal” excel headers, it opened the same way & now all workbooks on that machine also open with numeric column headers.