A black liquid crystal display is usually a sign of air contamination. Basically it's no longer a vacuum in there and it's not fixable short of replacing the display or module. (and it's almost impossible to get parts).
If it's just come into a warm place from a cold place then give it a while to settle down then try a battery just in case.
It's a lovely watch. the field effect ones don't go for as much as the dsm but working it'd be worth anywhere between 200 and 500 dollars (although ebay's a bit of a lottery recently so anything could happen). If it's broken then I wouldn't get your hopes up too much.