If you’re new to WordPress you should know that with each installation, WordPress installs a file called .htaccess among other files. A .htaccess (hypertext access) file is a directory-level configuration file that allows for management of Apache server configuration. It is typically placed in the root folder, and is able to override a subset of the server’s global configuration for the directory that they are in, and all sub-directories.
The original purpose of .htaccess—reflected in its name—was to allow per-directory access control, by for example requiring a password to access the content. Nowadays however, the .htaccess files can override many other configuration settings including content type and character set, CGI handlers, redirects, etc. The preference for the sake of performance is to use the Apache httpd.conf but most people use .htaccess as it is a text file and easier, more readily accessible.
Webmasters use .htaccess files to do many tasks including password protection, allow/deny by IP address or domain name, re-writing long URLs and 301 redirects. 301 redirects are very common as domain names change, old files are replaced by new files with different names. So .htaccess is used to guide users and search engines to the proper page. When a page cannot be found, the server usually puts up a 404 error page indicating that page cannot be found. This is not only disappointing to the user but also to the search engines and it may effect your rank.
However, many times when a .htaccess file is modified for a WordPress (or any other) website, errors can occur that renders the website inaccessible. It is safe to say you should always back up any file you want to edit or change. It is pretty un-nerving to see a “Internal Server Error” page specially for a large website or a blog with a large number of posts. Suddenly your blog or site is gone and no matter what you do, you cannot get it back.
Generally when this happens, you can still access your admin panel, if not you will have to delete the .htaccess file or its content in your server using your host’s file manager and then access your admin panel and of course your site will be available too. Now that your nerves have settled down and you are more relaxed you can put things back to the way they were. So go to your admin panel then settings then permalinks and save it. This action forces WordPress to install the proper .htaccess file back on the server. Be sure and select the permalink format you originally had.