Not familiar with that heater, but I suspect the sensor probably leaked. Did you check to see if the heater was still heating? Either way, I would unplug it and get it out of the sump (heaters are one thing I don't mess around with). I always keep a couple of spare heaters lying around in case of emergencies. If it's going to be a while before the replacement arrives, you might consider getting a cheap one from the LFS or Walmart until it comes in.
One other thing to consider. I run 2 under powered heaters on my 125. That way if one fails in the on condition, it won't heat up as quickly, and if it fails in the off position, it won't cool down as quickly. I also recently installed a separate controller. From what I have read, heater malfunctions are one of the leading cause of tank crashes.
Hope this helps.