The more time you spend with WordPress, the more likely it is that you'll run into an instance of the visual editor not working.  While it's possible to make all the necessary changes to your page in the html editor - the fix is easy enough there isn't any reason not to do it.

If you're looking to restore functionality to the visual editor, just follow these steps:

• Connect to your ftp
• Download (and backup) 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 );

• Save. Upload the new wp-config.php to your ftp. Now refresh your wordpress page and you should see the Visual Editor now.