MyBB Community Forums

Fatal error: Call to undefined method datacache::update_default_theme()
Website URL: http://bossforums.net/

looks like you can log into forum's admin panel.
can you run file verification tool available at tools & maintenance section of admin panel
to find changed / missing files ? focus is on php files. ignore files reported from install
folder & its sub-folders. you can also ignore images.
you have to replace the reported files from the source files pack of your forum's MyBB version.