one time i was installing some DENSO spark plugs on an eclipse and those things Ripped as I was tightening them in. ANyhow i pulled out the plug but a piece fell inside the block. When i started the car with a new spark plug on the piston came up and the rings on the piston somehow came off and it seized up!. Pretty bad scenario.

Anyhow be very careful make sure you get all of it when u pull the plug. If you broke it you should be able to use a Drill bit to drill into the plug and then unscrew it. they sell those at home depot

as for the threads. if the threads are still on the head they can be repaired if not working properly. Autozone sells thread repair studs that you screw in and out of the head to repair the threads.

try BOLT EXTRACTORS. they sell them everywhere. ebay sears home depot. i like the irwin bolt extractors the most from autozone