You have lost the password of any of the SQL Server service accounts. Or you simply need to change it.
Follow the steps below in order to change the password used by the SQL Server (Database Engine) service:
- Click the Start button, point to All Programs, point to Microsoft SQL Server, point to Configuration Tools, and then click SQL Server Configuration Manager.
- In SQL Server Configuration Manager, click SQL Server Services.
- In the details pane, right-click SQL Server (), and then click Properties.
- In the SQL Server () Properties dialog box, on the Log On tab, for the account listed in the Account Name box, type the new password in the Password and Confirm Password boxes, and then click OK.
The password takes effect immediately, without restarting SQL Server.