You might be able to thread a paperclip through it if there's still some sort of looped/enclosed metal to loop through. you might be able to get a new zipper (just the moving part), remove whatever keeps it from falling off at the top, put the new zipper on, and sew some thread around the top to keep it from falling off. Depending on the cost of the coat it might be worth paying a tailor to replace the zipper.
