fatal error after changing theme

hi,
after tryng to change my theme from serendipity to obscura,
i had this message:
Fatal error: Cannot redeclare class MostLikedPosts in /web/htdocs/www.qualcunoconcuicorrere.org/home/wordpress/wp-content/themes/obscura/includes/like-this/likethis.php on line 212

and my blog is down

can you help me, please?
Thank you