If you have ever forgotten to protect a worksheet in Excel, or if someone has accidentally unprotected a worksheet, you can force the worksheet to be protected again using the following steps:
- Open the workbook that contains the unprotected worksheet.
- Click on the Review tab.
- Click on the Protect Sheet button in the Changes group.
- In the Protect Sheet dialog box, type a password (optional) and click on the OK button.
The worksheet will now be protected again and you will be prompted for a password whenever you try to make changes to it.