If BUILTIN\Administrators has been left enabled you could try getting your account added to the server Administrator group temporarily to allow you to log in to SQL Server and then grant your windows account permissions.

If BUILTIN\Administrators has not been left in place then to recover the sa password you will still need to be added to the Administrator group on the server but you are going to need to stop SQL and start it in single-user mode e.g. net start mssqlserver /m this will then give members of the Administrator group access to SQL Server. You can then grant access and/or change the sa password.

There are a couple of articles on the web that go into more detail if you search for "recovering the sa password".