If this issue is occurring with one particular excel sheet, then it is because that particular excel sheet is protected. If the issue is occurring in all excel sheets, it is likely that you have a corruption in your excel settings. In that case use the following steps resolve the issue:
  • Close all open Excel windows
  • In Windows Explorer, go to directory "%appdata%\Microsoft\Excel" (without quotes)
  • Delete or rename the above file: Excel**.xlb --> Excel**.old
  • Now Excel will recreate a fresh setting file of the same name
  • open an excel file to check everything is working.