By default, do you mean the default theme (Twenty Thirteen or Twenty Fourteen) or the default state of your theme?
If you want to try & get your theme back, you can try downloading the theme’s ZIP file to your computer, unzipping the files, and uploading all the files to the theme’s directory using FTP. In the future, instead of making changes directly to a theme’s files, you should always create a child theme instead and make your changes there.
If you want to switch back to the default WordPress theme (Twenty Thirteen or Twenty Fourteen), I’ve done it by following the directions in this article. Please note that you should be comfortable and/or knowledgeable about how to use your host’s MySQL interface (phpMyAdmin or some similar interface). You’ll need to know the name of your WordPress database as well as the password. The information is stored in your wp-config.php file, in case you’ve forgotten it. You should do a backup of the database before making changes to it, in case you muck it up. Your host may have some sort of backup utility available (I know GoDaddy does, for example).
With whichever method you choose, your images and posts/pages should still be OK.
