If you're worried that a cat hasn't been eating or drinking, and it's not showing any signs of being starved or dehydrated, then try offering it a can of tuna water - don't buy the tuna in oil - and see if you can encourage them to drink that way. Not only the water will hydrate them, it'll also contain some much-needed nutrients from the fish. Otherwise, you should take the animal to a vet. The consequences of inadequate food and water can be dire.
Why? A cat who hasn't eaten for a while is considered to be starving, and starvation may have a damning physical effect on the cat's entire system. The same is true of dehydration, only the onset of dehydration is much quicker - and far more severe - than that of starvation.
Every organ in the feline body relies on water and the nutritional content of food to properly function. Every single one. Starved and dehydrated organs will steal energy and water from the rest of the cat's body, a process that slows as the cat's bloodstream becomes less liquid itself. When feline organs lack sufficient food and fluids to function on the most basic level, they dry up and begin to shut down. Not only could this send the body into shock, it might cause irreversible damage to the cat's organs. If a starving and/or dehydrated cat does not receive swift treatment, it may face permanent health issues - if it survives at all.
These are uncomfortable thoughts. They're scary. But it's important to state them as blankly as possible because you never want to take chances with a cat's intake of food and water.