Have you tried:
– deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switching to the default theme to rule out any theme-specific problems?
– resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.
– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?
– increasing the available memory?
http://wordpress.org/support/topic/253495#post-1017842
– reviewing http://wordpress.org/support/topic/411649
Open your wp-config.php and add this line of code at the bottom of the file before the “require_once” line:
define('CONCATENATE_SCRIPTS', false );
Thanks to: http://www.techiecorner.com/1656/wordpress-visual-editor-not-working/