If Your wordpress editor (one when you edit post/article from admin area) do not accept any text input or dont show any text, then there are some tips on how to discover cause/fix:

- best way is to check your Apache/httpd (webserver) log file
it may say something like: client denied by server configuration: /home/username/public_html/wp-content/plugins/counterize/counterize.js.php, referer: http://domain.com/wp-admin/post.php?post=495&action=edit
so here we know which file is denied (in my case by deny from all in .htacess file)
- do NOT add .php DENY FROM ALL into wp-includes folder or its subfolders .htaccess file

- try to upgrade wordpress from admin area
- backup site and replace wp-admin and or wp-includes folders files by ones in original wordpress script folder