We had an ex-batt like this, in fact we've had three or four!! Like Hairyhetty says, they often die, but we had one which we separated off from the others (we did this with them all incidently, even the ones that didn't make it), put her in the sheep trailer with plenty of warm hay - made sure she had plenty to eat and drink, we even gave her water by syringe into her beak to make sure she didn't dehydrate. She survived, went back to laying and became a very tame hen, we called her 'sick chick'. So persevere with her, it took a week or two before our sick chick was ready to go back with the others. However - beware because the others might not want her back and she might be bullied when reintroducing.