I also encountered this problem. Installed expose 4.6.1 two months ago and everything worked fine. No changes on my website. And in December 2007 no gallery view in the frontend. I can log in normally in the backend.
To me it looks like some security issue in the code. I'm not a programmer so don't know where to look. Is it possible that after a certain period you don't have no rights to view the gallery. I saw at the expose site that you can buy different licensees. Is it possible that it has anything to do with that?
Also uninstalled expose 4.6.1, renamed the directory under the components directory and installed expose 4.6.2. But also with the new installation no gallery view in the frontend.
When I do a clean installation in a new joomla site there is no problem.
Is there a solution to this?
Whats the link to your site? It might be a client change. Its hard to see why the frontend (which is just a SWF file for the most part) would just stop working. DOes it "Movie Not Loaded?"