I am sorry to hear that Vega is refusing to eat and drink and is very lethargic for the past 3 days.
It's difficult to know whether she is lethargic from not eating and drinking or her lethargy is part of the disease porcess that has led her to stop eating and drinking.
Of course without testing I cannot tell you for sure what is going on with Vega, 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 feel weak. 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. Is she spayed? If not a uterine infection (pyometra) could cause her symptoms.
Inflammatory bowel disease causes an inability to digest and absorb food, and the inflammation affects appetite.
Vega needs to see a veterinarian for an examination, complete blood count, biochemistry profile and urinalysis. In many cases this can point toward a diagnosis and direct treatment. But with many cancers bloodwork and routine radiographs may not be diagnostic. In those cases an abdominal ultrasound can be helpful.
In the meantime at home to try to settle her stomach and get her to eat you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These are acid reducers, are very safe, and may help her feel better. They can be used as needed.
You can also try a bland diet with increased fluids to perk her appetite and get her better hydrated. A homemade diet for this is 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat, and 2/3 boiled, plain white rice. Add low salt beef or chicken broth to her meals as well to improve palatability and get more fluids in which should help as well.
In an older girl with these sorts of symptoms it is likely she has some sort of serious disease process going on if she doesn't respond to simple supportive care. She needs diagnostic testing to further evaluate her.
Best of luck with your girl, please let me know if you have any further questions.