What version of Excel do you have?
From your question, the most likely cause is that the Workbook Calculation has been set to Manual. To set back to automatic, you would need to click on the Office button in the upper left of Excel with your file open. Click on Excel Options, in the left pane you will see some categories, you want to click on the Formulas category and new options appear in the right pane. Under the heading "Workbook Calculation", you want to select the radio button for "Automatic", click OK and formulas will now automatically calculate.
While in manual calculation mode you can determine when Excel calculates by pressing F9 to force Excel to perform all calculations on the entire workbook or Shift+F9 to calculate only the current worksheet.
Please let me know if that answered your question or I misunderstood what exactly Excel has disabled on your system/workbook.