If all that came out is the ceramic core you can reach back in and get the metal part which is the part that has the hex on it anyway. If the metal hex part came out with the ceramic core you need to go in with a screw extractor on the end of an extention and 12 point socket that will fit the end of the screw extractor. give it a tap with a hammer to seat it and then back out the remains of the sparkplug.