My rat won't eat or drink and is losing weight.
What do I do?

Rat is sick. Provide a low water bowl and feed as hand-fed baby. Consult vet.
In cases where a low water bowl only gets shavings kicked into it and is doing no good at all or the rat is too sick to use it. Try this. Mix 2 teaspoons of sugar into 4 ounces of water and feed (with an oral syringe) as often as you possibly can. Hand feed with anything you can possibly get the rat to eat (high-fat) as rats will lose weight extremely rapidly when severely ill. Definitely seek treatment from a vet.