When you last saw her, was it in the nest box? She could have been egg bound. But unless they are up and walking around it's hard to notice. Especially if they are nesting. It can often be mistaken for broodiness (long periods in nesting box). I always move a hen from a nest if she's been there a while and I suspect broodiness. If she is broody she will walk around all puffed up clucking and screaming at any boy (or girl) who comes near. She will also eat and drink and let out a giant broody poop. An egg bound hen will not poop like a broody hen. An egg bound hen may or may not be puffed up, and she will struggle to poop. If the egg is obstructed, she will not be able to do so, and often can prolapse. If the egg can be passed unbroken she should recover. Unfortunately if the egg breaks, she can and usually will develop an infection. I have had a hen die from this, even though I thought I got everything out. She was looking better and than she just died.
You can do an exam on your hen by just feeling for an egg in her abdomen - but best chances at knowing what is wrong is to open her up. Take pictures and send to Delisha, Mumsy or post here. We can all try to help you guess.
I know how you feel. I had a hard time imagining opening up a bird that was a pet. Unless I know for sure, I will do a necropsy on the next one.
I try really hard not to think of them as pets for this very reason... I'd open her up now, but I was upset and unsure if I would even be able to bury her (we still have a few inches of snow on the ground). So once I figured out I could dig a hole I just did it. I can butcher a chicken just fine (well, honestly the killing part bothers me, but once the head is off I'm good) but I wouldn't know how to begin to open a chicken up for a necropsy. Do you cut up the underside?
I last saw her in the nest box yesterday morning before I went to church... but it was morning, so being on the nest box wasn't a big flag. Then I went to church, checked on everyone when I got back (I can't even remember if I looked in the coop or not- I have an A-frame connected to the coop that is sort of an intermediary indoor/outdoor space where I keep their food and water- it's nice shade in the summer and keeps the snow off in the winter, and the chickens spend most of their time out there during the day when it's snowy, so a lot of the time I'll just peek in out there), then ran to the fabric store. She was dead when I got back later that afternoon.
I always felt she didn't mature properly. I don't have trap nests, but I try to watch as much as I can who lays what, and I never felt she laid regularly. She did go broody last summer. Her comb never got as big as the other hens. She's an orpington, and the only orp hen I have though, so I don't have anyone to compare her to, but my other single comb hens had much larger/redder combs. Hers stayed short and pink, like a chick's. I posted a pic of her on here once and voiced my concerns and everyone said she looked fine though.
Anyway, if anyone else drops dead at any point this spring I'll definitely "man up" and do a necropsy, or at least palpate for a stuck egg. I have a broiler chick who isn't going to make it... is it too much to hope that it will be the third and that will be the end of it??? I don't normally loose chicks- not even broilers.