I accidentally changed the permissions of sites/default/files folder to 777, but later I changed back to 755. From that moment images in the articles are not showing. It's giving 403 forbidden error. Please guide me.
Well, the question and description of the situation leaves some questions open but from a first glimpse I would assume, that you maybe missed to change also the nested folder and file permissions correctly recursively. Therefore read this Drupal guide and make sure that you also have a working temporary path set up in Drupal configuration to make Drupal able to rebuild its file system. Also make sure that you have cleared all the Drupal caches and Image style caches to force Drupal to rebuild the image styles and implementations.
BTW: theming is the wrong tag here. This is not a theming specific question.