Click the Office button and click the 'Excel Options' button at the bottom of the Button 'window'.
In Excel options, select the 'Formulas' tab from the list on the left.
In the section 'Working with Formulas' clear the check box alongside 'R1C1' reference style.
PS you haven't said if there was anything you did just before this change happened. Perhaps you have loaded an Add-In, or opened a Workbook from someone else or the Internet that contained a macro. Either of these could have made changes to your setup.