I lost one of my dear friends to a tumor last week. I had been syringe feeding her for a week before the exploratory surgery and put her down when the Vet. found how extensive the tumor was.
I do realize that that was an extreme case. Most cats will stop eating from time to time for many many reasons. The chance of some type of cancer is estimated to be between 1 and 4%. That is a 96 to 99% chance that it is not a tumor.
Tumor is the least likely cause.
Hairball is the most likely.
Put a glob (1/4 teaspoon) of Vaseline on her paw and she will lick it off. That might get things moving before she can get to the Vet.