Local admins can get into instance when sysadmin access has been lost by starting the server in single user mode. Here is how recover a sysadmin account without having to restart. Prior to 2008 BUILTINAdministrators was by default a member of sysadmin. In 2008/2008R2 that login was not added by default, but NT AUTHORITYSYSTEM still is.
psexec -s -i cmd
sqlcmd
create login [login] from windows
go
exec [sys].[sp_addsrvrolemember] 'login', 'sysadmin';
go