HTML Edit Area cannot show

I have been editing online documents in my opencart v1.5.6 admin area. We have 3 languages: Traditional Chinese, English and Simplified Chinese. I was experimenting different HTML to see the effect on webpage layout. However suddenly, when I choose to edit this specific document, I can only see the title fields, SEO fields, tag fields etc, but the HTML Edit area below is missing. I tried rebooting the server, or use a different browser and the result is the same for this particular document. This is the main page for one of our important content. We can still browse this page as a user, but now under admin I cannot edit this page anymore. Any clues how to fix it? Thanks a lot.