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.