Why is my cat suddenly lethargic and weak?

A cat suddenly becoming lethargic can be from stomach upset, a chill or cold, or depression. First, eating spoiled or toxic food causes gut discomfort and lethargy. Second, when a cat catches a chill or cold, its mood drops and it looks limp. Also, a depressed cat loses energy and reduces activity, seeming silent and unwilling to move.

But whatever the cause, a sudden change is abnormal; if you don’t know why, take it to a vet promptly.