can't access dashboard

You need to deactivate the plugin it is clearly not compatible with something else in your site.

Without access to the dashboard your options are limited, you can either delete or move the plugin out of the plugin folder on your web server via ftp.

or if you have access to the database through adminer or phpmyadmin you can view the wp_options table and find the row where the option name is active_plugins then delete it or set the option_value to ”
http://perishablepress.com/quickly-disable-or-enable-all-wordpress-plugins-via-the-database/

that will deactivate all your plugins and should let you back into your site.