The Number 5 cylinder has no compression

I was on my way home one Sunday, and after exiting the expressway the car starting rough idling. I pulled into a gas station to raise the hood to see if I could find out what was happening. I heard loud knocking, and rough idling. I checked for oil. The car had oil. I took the car home, but the next day I had a mechanic check it out and he said the number 5 cylinder is has no compression! I said what does this mean, his response was to replace the head or get a new engine. Is this true?