I would try to start your SQL Instance in single user mode. You'll need to stop the service first and then start it up again with the -m option. This should allow anyone from the sysadmin server role to login to the instance (but only one at a time). The command to start it up would look like this:
"%Path to SQL server%\MSSQL10.MSSQLSERVER\MSSQL\Binn\sqlserver.exe" -sMSSQLSERVER -m
Once it's started in single user mode try logging in with a sysadmin account and adding an account to SQL Server. Then start it back up again in regular mode and try to log in with the user you've added.
I found a link regarding this topic that I think might help. It mentions using single-user mode as well but seems to be tailored to a situation where your admins are locked out. Check it out here.