Fatal Error – Not able to login, post, or edit anything

I’m getting this error, which is preventing me from doing anything:

Fatal error: Call to undefined function twentyeleven_get_theme_options() in /home/kaylac/public_html/wp-content/plugins/menubar-color-changer/menubar-color-changer.php on line 25

How do I fix?! I’m very new and not sure what I did wrong. I do have a menubar color changer plugin that apparently has gone wonky, but I can’t even login to do anything about it.

I’m hosted through HostGator if that helps.

Please help!