If she looks unhappy and hanging by the nest, there is a good chance she has a slight egg-laying hiccup. The changeable weather doesn't help unfortunately plus her inexperience. You can bring her in to room temperature, in a comfortable box with straw, the warmth may help her lay. If you can get hold of some limestone powder for a quick calcium fix this should help, calcium not only in the main ingredient in egg shells but also helps the pelvic muscles of a hen work properly. Make sure she has access to plenty of fresh water, she needs to keep hydrated, particulary as we have had very changeable hot weather.
You can sink her in a warm bowl of water for a half hour, this will help her muscles relax, but only if you are confident with her and can keep her in overnight so she is warm and calm, not stressed. Take care not to squash her abdomen just in case the egg breaks, the likelihood is however the egg is soft shelled which is why she is so miserable and uncomfortable right now.
Keep a very close eye, for any other symptoms, for example bloating, tension in abdomen, lack of appetite and general malaise, this would then need a visit to a poultry experienced vet for further treatment and diagnosis. Good luck with her, light sussex are a really nice breed!