When I upgraded the User Online plugin (wp-useronline) a short while ago, I got a Fatal Error:
Fatal error: Call to undefined method scbOptions::get_key() in …/wp-content/plugins/wp-useronline/admin.php on line 56
I looked on Google and found the WordPress Support page http://wordpress.org/support/topic/396809 and renamed the plugin folder “wp-useronline” to “xwp-useronline”.
I then went to the Plugins page on my Dashboard, and then re-installed the plugin (by searching for “wp-useronline”).
Everything working now 🙂