We have a strict password change policy here, and having changed my password yesterday I could no longer start either of my two local instances of SQL Server.

On an attempted start I got an error message:
"The MSSQLSERVER service was unable to log on as with the currently configured password due to the following error:
Logon failure: unknown user name or bad password." error.

To resolve this I headed to Control Panel -> Services and selected MSSQLSERVER. I right clicked, selected Properties and opened the Log On tab. I updated the password and confirm password and then skipped back onto the General tab and started the service. It worked :-). I did the same with SQL Server (SQL2005), also successfully.