The possibilities are:
A. stuck valves, B. broken rings, C. blown headgasket, D. hole in piston, listed in order of likelyhood IMO.
Remove the valve cover and have someone crank it & watch to see if all the valves are moving an equal amount or at all. Then pull the plugs and see if they are covered in oil. If it will start you can check for the head gasket leak and also discolored paint stains on the head around that cylinder. You'll likely end up pulling the head for a thorough inspection and repair.
You didn't say how it was running before these problems and that is a huge clue.
