A lethargic kitten is not a good symptom. If your kitten is not eating and dehydrated then there might be something wrong internally. Bring your kitten to the veterinarian.
Many things, if the cat is between a newborn and 2 week old age, it may have gotten aspiration in the lungs, and if not treated right away, will die within 12 hours.
Could be a number of things, but perhaps not eating enough, or eating the wrong kind of food (or has eaten something that hasn't agreed with them). If it continues, take it to your vet.
