Sorry about the loss of your fish Betty.
I will keep this short but to be honest heaters are not very dependable at all anymore. Why this has been left to become such a problem is beyond reason. I have also had Aqueon Pro heaters fail in my tanks, one went hot and another just went cold. If you look at other forums this is a problem that keeps occurring and it is just plain wrong. I don't know what the answer is anymore, it's almost as if the heater manufacturers could care less about this issue. I mentioned this to an online retailer and they admitted that heaters are a weak link in the chain and it happens. Company's like Aqueon will replace your heater but what good does that do if it has all ready killed your fish ?
I have gone ahead and added heat controllers to all of my tanks so I don't have to constantly worry about my fish, they are expensive. I would recommend the Finnex HC - 0810m, they run about $35 each. I feel in some ways these company's are forcing us to take these precautions simply because they can't seen to build a reliable heater anymore. I know people will chime in and say this brand or that brand works better but there have been lots of problems with different brands in the last few years alone. I have been keeping fish since the 70's and the one heater that was the most dependable was the old green colored Jaegers, some of those would last 20 years or more in some instances.
I don't know what the real reason for this is, I mean aquarium heaters have thermostats built into them, why aren't they working ? So you will throw it out and buy a new one, that seems to be the real reason. People need to take this more serious and the manufacturers need to know that people are not happy being treated like cash cows. Maybe it's time for some public shaming via YouTube or in the extreme get people on board for a class action lawsuit to get this problem fixed once and for all.
There was a recent thread regarding situations just like this one over on a certain Tropheus forum but alas it's yet just another thread