The widgets are saved in the database and wp probably deleted the widget/sidebar data. You can try to restore the database if you’ve made a backup (sql file) of the database in the past. You can use tools like phpmyadmin to import the sql file. If you didn’t make a backup there’s basically no chance to recover the widgets. I’d recommend to install a plugin like: http://wordpress.org/plugins/backupwordpress/ to avoid this problem in the future.