so sorry to read abt your dear kitty, cancer in animals or us humans is a very hard road to travel....wishing her the very best
any steroid usually starts to work pretty quickly, like in a day or so...the inflammation is reduced and pain is lessened.
24 hrs isn't overly concerning to go without food or water...I would go another day and just watch her, if it goes beyond the second day than IV hydration may be needed. always speak to your Vet with these concerns and go with his or her recommendations
♥