My 3 year old female Lab is not interested in food and is lethargic, what do I do?
Whenever a dog loses interest in food, there either a medical problem or a behavioral one. The lethargy also may signify illness. She may have an intestinal viral or bacterial infection, or she may have ingested a non-food item or poisonous substance.
A vet visit, ASAP, would confirm if she's healthy, physically, and if she is (hopefully), this may be a behavioral problem caused by depression, stress, or anxiety. If there have been any changes in her environment recently, or she's been startled by a loud noise (thunder, fireworks, etc.), this could make her fearful and depressed. The vet could prescribe some medication to help, but that's usually a last resort, so try other recommended treatments, first. Comfort Zone for Dogs, with D.A.P. is a dog appeasing pheromone which helps to calm stressed/anxious dogs, and it can be found in most major pet supply stores and online.
Also, ask the vet for an appetite stimulant, if her symptoms have no medical basis. This will get her eating again. Slightly warm her canned food in the microwave, and the aroma should help awaken her appetite sensors.
I hope you're able to bring her to the vet today, and she'll be fine after diagnosis and appropriate treatment.