Have you tried re-uploading all the Admin files (specifically the index page)?
Sometimes corrupt files occur during ftp (and for other unknown reasons) and if you simply upload a "fresh version", it will get things moving again.
Just a thought, but I have seen it with "blank pages" before, and in every instance (so far) uploading new files (you do not need to install zen again as long as it was a good install the first time) has solved the issue.




