The droopy comb is normal for the Leghorn breed. But the droopy temperament is not, if it is unusual for this particular bird. How hot is it where you are? Heat can stress chickens out, and cause them to relax their wings and tail, giving the impression of illness. Other possibilities that she's egg bound or has Coccidiosis or Worms (even though you don't see any signs of worms, this doesn't mean your chickens don't have them).
Like Eggcessive said, check for a stuck egg. If she appears to be egg bound, soak her in warm water for 15-30 minutes to help relax her muscles. Then, dry her off the best you can and let her rest. You can also give her some calcium (in the form of a human calcium supplement, oyster shell, etc.), as that will strengthen her oviduct muscles.
If you feel no egg, then I'd consider treating for Coccidiosis and/or worms. Some good broad-spectrum wormers are SafeGuard, Valbazen, and the Worminator (contains Flubenvet and is what I use). You can usually find wormers at a livestock supply store, or you can purchase the Worminator online from here:
http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&products_id=584
Other than that, there isn't much you can do. I would isolate your hen, put some vitamins/electrolytes in her water, and keep her clean and comfortable.