That sucks, I had one break off like that years ago due to super heating in the cylinder from the turbo. Melted the threads partially to the head, plug broke off while I was trying to torque it out. There are multiple tools to try to remove it. Autozone, etc sell spark plug extractors and extractor kits. Another tool called an Easy-Out can be found at auto parts, hardware stores, etc, and is basically a tool that you drill/torque into the busted end of a broken plug/bolt/pin and then back out the original broken part. I think the threads go reverse on the Easy-out, or something like that.