I was recently found myself locked of a demo environment by removing the builtin\administrators group from SQL Server’s sysadmin role – a good practice by the way. I needed to get into SQL Management Studio.
This particular installation of SQL was running under the localsystem account. Using PSEXEC from sysinternals, I was able to launch a command prompt running as localsystem: psexec –i –s cmd.exe
I was then able to run SQL Management Studio (by right-clicking on the shortcut and pasting the path into the elevated cmd shell) and add my domain account back in as sysadmin =)