Did you have any chicks that hatched?Wow, heartbreaking!!! I'm speaking without experience here, but 12 hours sounds like a long time to go without a hen on the eggs... especially if it was overnight or inside an air-conditioned house?
I actually came to add my woes to the long list, we have a broody hen on day 11 or 12, and she is in a chain-link kennel to keep the others from bothering her, and inside that is a smaller wire kennel that we lock up at night so nothing can get her, as the chain-link kennel does not have a predator-proof top to it. Anyhow, I'm out of town and hubbie is taking care of things at the farm for the week- he opened her up this morning and all was well. Went to close her up tonight and she had knocked down one of the sides of the nesting box (hard to describe the setup), and was sitting on top of it, on top of the eggs. Nothing had broken, but the eggs were cold to the touch he said. No clue how long she was technically off of them... so sad and frustrating... It's luckily warm weather at home, but if she was off the nest to eat/drink/poo this morning or afternoon, then it could have been nearly all day that she was sitting on the board over her eggs... ugh. I just want to cry! Can't do anything about it now except keep my fingers crossed and re-rig the nesting box so it can't happen again, but I will be so devastated if they don't hatch. Out of 12 dorking eggs I had under her, I had just candled 2 nights ago and had 9 definite viable eggs and 1 that was possibly viable! It's always the longest 3 weeks of my life, and now, since this happened, and because I'm out of town, it will be excruciating to wait and see!!!!