you can try to put a teaspoon of heavy oil into cylinder 5 and redo compression check. If still zero reading... then it's your valves. If it begins to show some compression, then u have stuck or broken rings
possible broken piston rings, a blown hole top of the piston from hot spot and prolong pinging, one way to check is to just pull the oil filler cap while it running ( prepare to get sprayed) if its a blow by from the piston, possile collasps camshafter lifter, bent valves. what ever it is will be major!