Yikes is right! Can you tell if there is a foreign object in the cylinder? Like if you shove a probe in there, can you feel anything loose moving around? Maybe turn the crank a bit so the cylinder in that piston drops down a little? If you can fix a magnet to a piece of wire and lower it through the plug hole, perhaps you can retrieve anything small enough to pass back through the hole. Assuming it's ferrous and that there is actually something in there, which is all speculation.
One of my greatest fears during the head gasket job was dropping stuff into the engine during reassembly. Despite using masking tape and rags to cover all points of entry, I still managed to drop a fuel injector O-ring into one of the cylinders after replacing the heads. Thankfully I was able to vacuum it out without having to take stuff back apart.