fatal error

I dont know why, what I just tried to set up is to add the app ID (key and secret) for my facebook. (I am not quite sure of how to develop an app for facebook yet)
It seems that, after I added the key and the secret of the app, my website suddenly went down.

The message is :
Fatal error: Cannot redeclare class Facebook in /home/html/my.domain/wp-content/plugins/wordpress-chat/lib/facebook-php-sdk/facebook.php on line 93

I tried to delete the plugin of wordpreses chat folder, through FTP, yet still showed above message.

Please advise what I should do, in order to have my previous setup back ASAP..@@..

Thank you.