A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Activate the Formulas tab of the ribbon. Make sure that Calculation Options in the Calculation group is set to Automatic.
Take a look at the status bar at the bottom of the workbook window. Do you see 'Circular References'? A circular reference is a formula that refers (directly or indirectly) to the cell that contains the formula. In most situations, this is undesirable, so you should correct it.