I did exactly the same on one of our cars a couple of months back. I fished it out using two long, thin screwdrivers. It hadn't melted.

 

You could just remove the spark plug and slide the rubber off and put it back in the socket. Then reinstall the spark plug and make sure the rubber comes out with the socket this time.

 

I had to reglue the rubber insert on my plug socket because it always did that. Quite annoying. It's not damaging anything staying on the plug as long as the coil wire goes on completely, but a pick with a small hook on it or simply remove the plug with the same socket and don't use the rubber cushion on installation.

 

They do sell a magnetic spark plug socket that doesn't use a rubber. Pretty cool.

 

You could glue the socket and slide it over the rubber,let it set then remove it.