Just had this same problem with SharePoint 2007 and posting my solution for anyone else that might come across it.
We have a Document Library with 31 metadata items - yes 31! Have been using this library for months with lots of documents all with metadata populated. Suddenly all the fields, except for Title, disappeared from the Edit Properties form. Luckily the metaddata wasn't lost as you could still see the metadata in the Document Library views, but couldn't edit them as they didn't appear in the Edit Properties form. For Word documents, open in Word 2010, showing the document properties panel only shows Title. BUT, now all metadata field show as Custom properties (as viewable in Advanced Properties dialog).
Upon investigating I found that SOMEONE had created a Content Type of "Document" for this library - which we didn't have before this problem- AND only the Title field listed as properties. I couldn't delete this Content Type getting a message "document already exists" so decided to make the Content Type fit my needs. Here's how I did it.
In Library Settings > Advance Settings > "Allow management of Content Types" set back to "Yes". Then, in Library Settings, under Content Types clicked "Document" to open the settings for the content type. Click on "Add from existing site or list columns". In the Content Type settings form, the "Select columns from:" dropdown list is defaulted to List Columns (which listed all the Document Library metadata fields that I was missing). Selected and added them all. Then went to change column order for the Content Type as they were before.
They now appear in the Edit Properties form (and Word Document Information Panel) as desired.
Hope that helps someone because it wasted a day of my time.