There are many reasons why your hamster might be lethargic, not eating, or not moving around much. The best thing to do is have him checked out by a vet. However, if the cage is poorly ventilated or dirty, it could be the hamster's own urine that is making him sick, and as soon as he goes back in the cage his symptoms will reappear. It’s important that your pet's cage is well ventilated, doesn't smell, and has safe bedding. The wrong bedding can cause respiratory problems in your hamster.