Hello there and welcome to BYC!
Sorry about your hen. Sounds like she probably is not egg bound since she laid yesterday. You say she is very light? Is her comb pale too? Have you ever wormed your birds before?
Being thin can be caused by her either not eating caused by some ailment or worms eating her food inside her intestines. Have you seen her eat a lot of food and still she is thin? Or have you not seen her eat in a while over the past weeks?
If she has been eating like she is starving to death and still losing weight, I would suspect worms as they will eat all the food she eats.
If she has not been eating or drinking much over this past week, I might suspect Coccidiosis. This will cause the bird to go off her food and water. She might look all fluffed up and very lethargic.
So if you have never wormed your birds, you might try this first. If that doesn't do it, you might head to the Corid for curing the Coccidiosis.
Give her a really good over all exam. Feel her abdomen for a water belly. Check her legs and feet for any heat or swelling. Pads too. Any respiratory issues. Anything that might be ailing her.
You might also post this in our emergency section for more input on this....
https://www.backyardchickens.com/f/10/emergencies-diseases-injuries-and-cures
I hope you can get her back on her feet to good health soon.