so we've had this problem before during the summer time our white crested female duck wouldn't lay and put her legs out behind her and couldn't move them so we put her in some water and she would go crazy drinking it and then would get better within an hour but now our mallard has that same problem but its going on for THREE DAYS
P.S they are housed with chickens and a mini potbellied pig
and Welcome To BYC! Sorry about your duck, TwoCrows gave you some links that will hopefully help you figure out what is wrong with her.