It sounds like you may have inserted some CSS code that has broken or hidden the display of the WordPress dashboard.
One solution would be to use FTP or SSH rename the css-manager plugin directory to something like css-manager-old. Once the plugin is disabled, that will stop the custom css code from getting inserted into your website, and should fix the problem. Next you could use phpmyadmin or a similar DB admin tool to edit the wp-options table, you will need to look for the setting named: "custom-css-manager-settings".
Feel free to contact us directly via our website if you require further assistance.