Hello tilehelperdan -
Sorry to hear about your heater troubles. This winter has sure been a cold one.
You will want to get that other heater running as soon as possible to keep a hole in the pond to "vent" out the gases that you were able to smell.
I also strongly encourage you to get an aerator rated for outdoor use. This will add needed oxygen into the water and help the pond "vent" out the toxic gases.
Back then the winters were warmer (not last winter, but the winters before that) people used to get by with having just a heater. Now that winters have been normal (cold again) i think a small watt heater and aerator our your best bet.
A water change is not a bad idea to get the bad water out. Around 30 -40% change should be good. Be sure to add de-chlorinator if you live in the city.
What type of heater were you using that burned out? I have been keeping tally of the heater models that stop working.
I also do not recommend the use of cattle tank heaters for they can cost over 100 dollars a month in energy to run them!
Hope this helps!
Keep us posted,
Jake Langeslag
The Pondologist