when trying to get into wp-admin site or me is: Fatal error: Can not redeclare c

I installed a plugin and can not do me nothing, nor can I get into wp-admin, what should I do now, where to turn and who to adreses me with this problem?
when trying to get into wp-admin site or me is: Fatal error: Can not redeclare class wpdb in / home / pleasene / public_html / wp-content / db.php on line 54.
thank you