I was having this problem. One of my plugins was causing an error. Using Firebug, I turned off all of the styles on the page. Then I saw something like Fatal Error: get_plugins() called from /plugins/WooDojo/file.php in the content of the page. So I disabled the WooDojo plugin and the problem went away.
Mayber re-check plugins that interact with your header. It’s possible that there’s an error causing it to stop displaying.
I don’t know if there’s a more recent thread about this – but here’s what drives me crazy – they developed a UI that doesn’t do anything. If you go to the media library, click Edit on an image. Click the Edit Image button. Drag the mouse cursor around the image – a small box appears, and the pixel amounts change in the Selection text boxes. Under “Apply changes to:” select “Thumbnail”. Click the Update Media button. What does all this UI do? Absolutely nothing!
Why did the developers spend all of the time to create this UI if it isn’t supposed to work? I’m on 3.2.1. How’s anyone supposed to know that they were just joking around about editing thumbnails?
