That sucks, how long had it been unplugged? I lost a fish in a similar manner although my heater didn't stop it just was not strong enough for the icy winter basement, I was not checking the temp every day so I didn't notice until a fish died.
How cold did your tank get before you noticed?
I thought it was warm on mars... only kidding of course hoping to lighten mood.
What I do to prevent that, as I have had the same issue; is to have my water pumps and heater plugged into the same power strip. This way I turn off the power strip to do the change, that way I can't forget to turn the heaters on.