Doing water changes with some heaters can be dangerous. Most say right on them "Do not use dry" and it isn't referring to your state of innebriation
If you leave them on while doing a water change it can cause them to overheat and melt/blow the thermostat. Just a warning for future water changes.
Most stores will take it back as "defective" as long as you can provide the packaging and reciept, otherwise you can try
contacting topfin customer service, tell them it stopped working and see if they will fix it or send a replacement. Contacting the manufacturer can be hit or miss and sometimes they demand you send it to them first to fix but you also have to pay shipping. Either way it's worth a shot but if you end up shelling outmore cash, just buy a new heater.