How to re-transcode sequence Encore CS4?
It sounds like your Premiere project is missing assets.  When you embed a Premiere project in to Encore, it will actually run a render in Premiere when you go to encode it in Encore.  If the Premiere project can not find assets that are used in the Sequence that you have embedded, you get a "media offline" image.

I would suggest opening the Premiere project and verifying that it has proper access to all the source files it needs.  I'd also verify that the Premiere project link itself is still intact.