If your rat is dehydrated and refuses to drink, warmed fluids must be given under the skin by injection. One cause of lethargy is pesticide poisoning. This will also cause the pupils to be constricted so they do not expand in the dark. In this case you'll need to rush your rat to the vet for an injection of atropine. Electrical shock can occur when a rat chews through a power cord.
Electrocution can cause unconciousness and can also stop breathing.
Place your lips around the rat's nose and blow into her lungs. You will have to blow fairly hard, enough to make the chest expand, but not too hard. After blowing 2-3 times, check to see if breathing has started. If not, firmly massage the chest for a couple of seconds and then blow in their nose again. Alternate the chest massage with blowing into the lungs.