Is this just a problem with one instance? If so, in the feature settings, make sure that "Use WYSIWYG Editor?" is checked. If that's unchecked, you are just editing the raw HTML code.
If it's a problem with all HTML instances, it could be that the version of the editor you've chosen is missing on disk. Take a look at the editor BasePath keys in web.config, which look like the following examples, then make sure the specified location exists.
<add key="CKEditor:BasePath" value="~/ClientScript/ckeditor365/"/>
<add key="TinyMCE:BasePath" value="~/ClientScript/tiny_mce357/"/>
You should also go through the steps in Basic Troubleshooting to see if an error is getting thrown in the log.