Joomla upgrade and T/S existing errors

We have a virtual server that hosts many joomla-based websites. On one of the sites, the administrator backend now produces just a white screen. The error logs yields two structure errors that have been there for a year. The contact page, which we no longer have proper management access is now a gateway for unauthorized emails of the SMTP (exim) server that is resident on the same server.
We need the site to be restored to fully operational status with all errors resolved and to lockdown the leaking of the internal user using the SMTP to distribute spam.