There is a chance that you may have accidentally touched a combination of keys, and it might have disabled the Focus Mode feature. To disable the Excel Focus Mode, press the combination keys Alt + V, ...
How-To Geek on MSN
Excel's hidden formula trick: Alt+Enter shortcut revealed
The Alt+Enter shortcut isn't limited to new formulas you write—you can go back and clean up any messy formulas at any time.
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