SQL Server authentication failed after server promote we recently promoted a Windows 2008R2 server to domain controller.
We then found that it hosted a SQL Server 2008R2 Express database instance.
If we try to login to the DB with the domain admin credentials we get a login error 916.
We are also unable to enable the 'sa' user due to error 15151.

Any help will be greatly appreciated.
Alex.
Found it!
I was able to create a privileged user while putting the DB in single-user mode.
Here is the link: http://v-consult.be/2011/05/26/recover-sa-password-microsoft-sql-server-2008-r2/