Sorry to hear of your loss.

It's very difficult to know what a garter dies from, even a necropsy can be inconclusive. Also, your parents were quite sensible (whether intentionally or not) because if you're still trying to work out what killed your snake you shouldn't consider putting another one into the same tank. The tank should be deep cleaned, I'd be using kitchen cleaner, steam and lots of water to rinse.
It sounds like you should address some heating issues, the others have given good advice. Once you have sorted the environment you should be good to look for a new garter from a reliable breeder.