I've started a ticket with my host too, however wanted to get some information here regarding what could be the cause. My site was down 30 minutes back with a "connection refused" error, after it came back I am unable to access admin.php with "The requested page could not be found." being returned as the error (the regular error page that xenforo serves). I tried re-uploading the files, however that didn't help. Logged out, logging in - refreshed my cache. Nothing helped. Disabled all add-ons via config.php and even then it did not work.