There are two possible causes for this.
1) You have the visual editor disabled in your WordPress settings. Make sure you don't have the visual editor disabled. If that is not the issue, then...
2) You have PHP version 5.3.9 or higher and are hitting a PHP system limit for maximum number of post input variables. You need to increase the max_input_vars PHP variable. By default it is set at 1,000, so you should increase it to something closer to 3,000 or even 5,000 (or higher as needed). Add php_value max_input_vars 5000 to your .htaccess file. If you need help doing this, contact your web host - they can help you with these kinds of changes.
If you have increased this variable and you are still having the issue, there are a few other variables you will need to update, including but not limited to: