Hi Stef and welcome from me, too.
There's no such thing as a lazy chicken. If she's lying down and is immobile, it's bad news in afraid. Chickens always try to stay on their feet because the rest of the flock will attack any that are showing signs of weakness and illness, so actually, by the time they get to the lying down stage, they're usually pretty poorly.
As Chris says, age is a factor - especially in hybrids who have laid regularly and frequently, things often go wrong with the egg laying equipment in older hens, either peritonitis or egg binding are both common. If peritonitis, this is invariably fatal, also very painful, so you might consider an urgent visit to the vet if you want a more definite diagnosis - of course, on here we can only give general advice based on our experience, having not seen th bird and since we are not vets.
Another possibility is a very heavy worm burden. Have you ever wormed them with Flubenvet? This is the only licensed wormer for poultry and should be used at least every 4 months, from the age of 20 months. All chickens get worms and very heavy worm burden can pull a hen down in itself, and also will affect her resistance, so other health problems are magnified. So-called herbal gut tonics are not effective at killing worms.
You may be interested in the articles on worming on the main Poultrykeeper website, see links at the bottom of the page, particularly click on the Roundworm link to see what they can do to a hen's gut.
http://poultrykeeper.com/general-chickens/worming-chickens
