First try visiting the dashboard directly at https://mysite.example.com/wp-admin (with your site's URL). If you get in you can deactivate the plugin.

You can forcibly deactivate and remove a plugin by using your cpanel file manager (or other file access) to delete the plugin's code. This is useful if a plugin breaks your dashboard.

Find the root directory of your WordPress installation. It contains a bunch of php files including wp-config.php.

From there go to the wp-content subdirectory, then to the plugins directory. There you will find a subdirectory for each plugin. Delete the directory for the offending plugin and all its contents.