The water probably fell below because it got too hot, which led to faster evaporation. This probably resulted in a positive feedback loop: Think - ice reflects sun, sun melts ice, leading to less reflection, leading to warmer oceans, which melts more ice, which leads to more heat entering ocean, on and on.
I'd get a new heater honestly. I've only had one heater malfunction and go up to 90 in my ten+ years of keeping fish. I'd be worried that it will malfunction again.