To convert to .idml (which I presume is what you mean) the file must open. There is, however a "blind open and convert" script that bybasses the user interface and sometimnes works that you can copy from Kasyan's site at http://kasyan.ho.ua/indesign/all/blind_open_and_idml-export.html
if that fails you will either have to bite the bullet and pay (and Markzware is the best bet), submit to Adobe and wait, possibly forever, to see if they can recover it, or rebuild from scratch. You'll have to weigh the relative costs to decide.
... View more