you should separate that bird form the others and monitor her closely as this is a serious sym,ptom. Has she been regularly laying before this? Have you tried placing her in a warm bath in case it is eggbound? (use the search function here and type in EGGBOUND >lots of threads on this)
I had the same thing happen to one of my girls. I was away on vacation and came back to something resembling a penguin. Her abdomen was huge and firm, with her butt near the ground(thus the penguin stance) I went inside to do some research and came back out with gloves and lube to check for an egg. I didn't find anything! I did happen to look in the nest area and saw the biggest egg I've ever seen. Maybe having mommy home again relaxed her enough to pass it.
She seemed to perk up almost immediately, but it took over a week for her stomach to shrink back up. She seems genetically predisposed to lay these gigantic eggs. I don't think she'll have a long life. Kids are just too hard on her...
I do remember reading about something else that can cause the penguin stance, but can't recall. I'll try and find it.
I'd start by probing for an egg. Just be careful - you don't want to break it inside her.