Liz, I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. Please let me know if you still need help.
Unfortunately, a droopy comb (dehydration) and not laying can indicate any number of illnesses or health issues in chickens. In avian medicine, there's rarely one cause of such a presentation, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. Necropsy of a newly dead or a sacrificed severely ill bird then refrigerated (not frozen) can be an important diagnostic particularly in large flocks. If this is the case, your best course of action is to reach out to your county-extension poultry personnel or avian-oriented veterinarian (please see here: www.aav.org) for help in having a necropsy performed.
It's best to approach the diagnostic process with a clear sense of her financial value to your operation. Although some services such as your county animal disease diagnostic laboratory might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one hen. She may or may not be suffering from the same cancer as your other leghorn you had to euthanize. It frustrates me that I can't be more specific for you but such is the dilemma of poultry owners and vets alike.
Have any new symptoms arisen that might help clarify why she's acting ill - conjunctivitis, a change in her eyes, nasal discharge, sneezing, coughing, gaping, increased respiratory rate, regurgitation/vomiting, diarrhea, swollen coelom (abdomen), a change in posture, or lameness, etc?