I am sorry to hear that Cookie has had rapid weight loss and is now refusing to eat or drink.
Of course without testing I cannot tell you for sure what is going on with Cookie, but I can give you some possible reasons for what is going on with her.
Organ failure (kidney or liver disease), or ketoacidotic diabetes can lead to a build up of metabolic wastes that lead to nausea and a loss in appetite and an inability to process the nutrients she does take in.
A gastrointestinal foreign body, or an abdominal mass/tumor placing pressure on her gastrointestinal tract can lead to loss of appetite because she feels full. If she isn't eating normally she will lose weight. And cancer increases an animal's metabolic rate, thus burning calories at a faster rate, and can cause increased amounts of metabolic waste and inflammation, both of which affect her appetite.
Systemic infections can also increase her metabolic rate, burning calories faster and the toxins from an infection will cause a decrease in appetite.
Inflammatory bowel disease causes an inability to digest and absorb food, and the inflammation affects appetite.
Cookie needs to see her veterinarian for an examination, complete blood count, biochemistry profile and urinalysis. In many cases this can point toward a diagnosis and direct treatment.
Best of luck with your girl, please let me know if you have any further questions.