My cat had loose stools after eating thawed canned food and is now lethargic with no appetite

The loose stools are likely gastroenteritis from the sudden canned-food change — new cans, like new kibble, can upset the stomach. Fast the cat for half a day, then give an anti-diarrhea remedy; once vomiting and diarrhea stop, offer small amounts of softened kibble mixed with active probiotics three times a day to aid recovery and protect the gut. If diarrhea continues, keep giving the anti-diarrhea and gut anti-inflammatory meds twice daily, and feed nothing else. If the cat becomes lethargic and refuses food, suspect parasites or viral enteritis and take it to a vet promptly — prolonged diarrhea and no eating cause dehydration and electrolyte imbalance that may need fluid therapy.