It would have been best if you posted the entire error message but by speculating what could be happening is that your SA account is usually in denied mode since the time of the installation has finished, you need to login as a local administrator who has access to the SQL Server instance with sysdba ..> this would have been done during installation.

Also check on the server properties if mixed authentication is allowed, you would never be able to login if this is not enabled.

Regards,