I inherited a domain with a SQL Server where nobody knows password to it's admin login. I have full rights on the domain and that server, is there a way to get access to the SQL Server instance?
By default SQL Server 2008 keeps the Built In Administrators group out of SQL Server access wise whereas in 2005 and below the Built In Administrators group was automatically granted SQL Server sysadmin rights.
As a local administrator you have a couple options though: Restart SQL Server in single user mode (as described in this post: http://deepakrangarajan.blogspot.com/2008/01/forgot-sa-password-in-sql-server-2005.html)
Or without restarting SQL Server, this approach also works and is a bit easier: http://sqlblog.com/blogs/argenis_fernandez/archive/2011/07/10/think-your-windows-administrators-don-t-have-access-to-sql-server-2008-by-default-think-again.aspx
I've had to rely on both approaches and they've each worked. The second method is my preferred when I need to do it.
When you get it back up consider creating a Database Administrator AD group and granting that access to the SQL Servers in your environment. Then you can move your DBAs into and out of the group.