if it has no compression on one cylinder it is either valves or rings stuck. I assume you have determined what cylinder is at fault. You could try to soak the cylinder in some type of high detergent lube , clear the cylinder of it and run it kinda hard to get the temps up, But be careful if to much unburned fuel migrates to the oil in the crankcase ,that could be very bad. I do not know what engine you have , is there a inspection cover or a way to access the lifters so you could look to see the valves move on the bad cylinder if not I guess you could pull the head. good luck
