First of all, I'd suggest you to look at page source, or, if you can, with a javascript console. I have had many trouble with HTML editor in moodle, and most of them were related to server-side problems (like advertising robots) and I only discovered them after reading page source.
The second step would be cheking for missing files (I have had some trouble with ftp clients aborting before sending all files) and permissions (make sure your lib folder is readable).