Updated, fatal error

I recently decided to update my modules after a long time. I updated the Metatag module to 7.x-1.0-beta9.
However my drupal core version was 7.12. Now when I try to access any of the webpages, I encounter a fatal error:

Fatal error: Call to undefined function entity_language() in /home/*/public_html/sites/all/modules/metatag/metatag.module on line 1844

This I understand is because the Metatag module requires the drupal core to be version 7.15 or recent as the function entity_language() is not present in the versions before it. I tried to disable the Metatag module, but am unable to do so (I cannot uncheck the enabled box). Now, I only have access to the admin panel of the site. Short of attempting to upgrade drupal core or restoring the site from a backup is there anything I can do? Is there any way to roll back the update to the Metatag module or uninstall it?

I do not have a lot of experience working with drupal. Please help!