If Fluffy is cold to the touch, and/or sluggish, then she has had a hibernation attempt, and must be gently warmed up to normal body temperature (20-30degrees Celsius) as soon as possible. She will continue to be sleepy/sluggish after a hibernation attempt. Her immunity will also be less active, so leave her in her box (except for warming up if using body temperature) until she displays signs of her usual curious, eating and drinking self.
To prevent hibernation attempts overnight, when the house is coolest, we have a microwavable hot water bottle to leave in her tank, under her sleeping bag. The heating instructions are on the pad itself and it stays warm for about 10 hours.