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:

1. Open the workbook that contains the unprotected worksheet.
2. Click on the Review tab.
3. Click on the Protect Sheet button in the Changes group.
4. 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.