Windows Administrator doesn't have permission on SQL Server

My previous system administrator created a SQL Server 2008 instance on my local machine with his domain User. Now, I cannot create, delete, or view any databases or tables on that instance. Even with my domain User, or Windows local administrator. It says "CREATE DATABASE permission denied in master". The users section in Management studio shows only two users 'BUILTIN\Users' and 'sa' which is disabled. 'Builtin\Users' has only public role. It seems no user have permission to this instance. How can my Local Administrator get permission to it ? I cannot delete the instance as it has important databases.