Was she a first time hen? Sometimes, but rare and does happen, a broody hen will starve herself to death on the nest because she doesn't get up to go eat, drink, or deficate. Was her body really light?
It is possible that she starved/dehydrated. I almost lost my Buff Orp, Sunny, the first time she went broody. I was having to remove her every single day to eat and drink and she wasn't sitting on anything. We tried to break her up with all the usual tricks, but nothing worked. About Day 19, she began to twitch her head, lost her balance and completely quit eating and drinking. We had to perform emergency measures, syringed vitamin-charged water into her mouth, vitamin E squeezed into her beak daily, then finally, about Day 23, she suddenly snapped out of it, very much thinner than she started. If we had not been very observant, I believe, and jumped in to save her, she would have died. Most will not go overboard like that, but she just was not normal during her first broody spell. The second time, she hatched out two chicks and every day, got off the nest to eat and drink on her own.