Twitter stopped using old API, in order to have have your twitter feed working you must to the following steps.
1. Make a backup of your class.shortcode.php file located in Clapat folder of your current Eleven-WP Theme Installation
2. Edit class.shortcode.php with your favorite text editor (recomanded notepad ++), find function twitterFeed - for 1.2 version starting from line 15 till line 77
3. Replace those line, with the content from attached .fix file. - http://clapat.ro/themes/eleven-wp/fix.txt
4. Upload the modified file and hard refresh the page. (CTR+F5 for Chrome)