Sorry to hear about your sick girl. Could she be egg-bound? You could try giving her a good dose of castor oil and see if it helps. A good dose of garlic in the mash can help too.
Five years isn't that old for a chicken -- I had one live 16 years and she was still a professional chick raiser, and she was also very good at eating snails, slugs and other pests and training the young 'uns to do the same. But it depends on the breed -- some of these new breeds are 'designed' to be terrific egg layers for a few years, but then their organs shut down. I prefer the old breeds for that reason.
A slow cooker is perfect for cooking old birds, but I wouldn't eat a sick bird. It's better to bury her instead if she does die and you don't know what is/was wrong with her.