(I see that your situation has resolved itself, but I want to talk a bit about cat anorexia and fatty liver disease because it can be a real problem).

Cats can stop eating for a variety of reasons. They ate a bug, they have a hairball making them feel weird, they're stressed, they have a cold, there are a ton of reasons.

**Feline Hepatic Lipidosis**

The main danger when a cat stops eating is [feline hepatic lipidosis][1], commonly called fatty liver disease. 

> The exact mechanism by which fasting in a previously healthy cat
> causes hepatic lipidosis is not clear. The process is unique to cats
> in both severity and rate of occurrence. It is understood as the
> anorexia continues more and more fat is broken down throughout the
> body and that this fat is then transported to the liver. The liver
> should then process this fat and export it back to the rest of the
> body in a new form. In cats developing hepatic lipidosis this process
> is impaired and fat accumulates in the liver. Damage to the liver
> occurs as a result of liver cells being too swollen with fat.

It's also not really well understood how long it takes for a cat to develop fatty liver disease, but it's believed that overweight cats will develop it faster than normal/underweight cats.  

I had a cat with recurring anorexia (as a result of sinus problems) and the general timeline we followed was:

 - Day One: Offer Food, no pressure 
 - Days 2-4: Initiate Force Feeding (high caloric density soft food with a syringe every 2 hours) 
 - Day 5+: Hospital stay for regular liver monitoring (hospital staff continued
   force feeding)

Fortunately he never developed fatty liver (he was pretty underweight from these recurrent episodes). It's very difficult to get enough calories into a cat with a syringe, and if I knew how often we'd be doing it I would have had a feeding tube placed in his neck to aid in the feedings.

Feeding tubes are a low stress treatment method for chronic anorexia in cats. Fairly recently, Juliet stopped eating due to stress after surgery. She hates being handled by people, so I had the vet put in a feeding tube immediately and she was fine a week later. I believe that if we had tried to syringe feed her, it would have continued to stress her out (continuing the anorexia).

**Dehydration**

Another concern when a cat stops eating is dehydration, because cats get most of their water through their food (especially on a wet food diet). There are two ways to check your cat for dehydration at home.

 - Scruff test. Pull the skin of the scruff (the back of the neck) away from their body a short distance and observe how quickly it returns to place (a healthy cat should snap back immediately, a dehydrated cat will sort of slide back into place).
 - Gum test. Press your finger on the cat's gums. When you release your finger, there should be a white spot where your finger was. In a healthy cat, it'll take 1-2 seconds for that spot to return to pink. In a dehydrated cat it will take longer.

Dehydration can be easily treated with subcutaneous fluids, but a vet needs to examine the cat to be sure that there is not an underlying problem causing the dehydration.

**When to See Your Vet**

If your cat is dehydrated, you should see your vet immediately.

If your cat has not eaten for 2 days, you should see your vet immediately.

If it's a Friday and your cat just stopped eating and your vet isn't available on the weekend, call you vet and ask for advice specific to your cat's health and any existing conditions.

  [1]: http://www.acvim.org/PetOwners/AnimalEducation/FactSheets/SmallAnimalInternalMedicine/HepaticLipidosisinCats.aspx