1994 Ford Ranger Engline Block Heater?

I realized for the first time in two years that my truck had an engine heater. (First time it was plugged in I smelled oil, which should be normal). During the cold weather -0*F, I plugged in my Ranger again, only to quickly unplug it because as soon as I plugged it in, somewhere by the heater lit up blue, threw sparks, buzzed loudly, and had a puff of smoke from the area (Looked like a welding arc). The truck still starts and runs, would there be any explaination for it. Shorted out, fried the heater, or ice on the connection? I am a little worried to try and plug it in again, possibly causing a fire. I was using and ext. Cord with the proper ratings. Thanks.