Misfire code and compression problem

Something is wrong with the engine. Leaving the gas station after a long drive and the car starting losing rpm’s and shaking. It shut off. Starts up barely but will not idle. Engine sounds terrible. Mechanic is saying one bank of the engine seems to be misfiring and no pressure in one of the cylinders. He said when he pulled the spark plug the plug was damaged. It seems odd to have a major engine malfunction like this when I was only going 25 mph.