heater blip

Main tank got too cool yesterday. I forgot the water in pipes probably still near freezing, and guess I added too much colder water with the 50% wc. It felt cooler to my hands than norma, when pruning plants, and some fishes were hanging around suspiciously still, then I looked at the thermometer surprised find it was only 72°and the heater wasn't kicking on. Can't see the light well because plants now grow up in front of it, even harder to see numbers on the thermostat dial. I'd have to lift it out of the tank to check it and lacking time, floated a bottle of very hot water (could see the heat waves emanating off it in immediately surrounding tank water) and that helped it warm up a bit more. Later in the day unplugged the heater, plugged it back in and now it triggers on. Hope this blip doesn't mean it's faulty, I've had it just two years!