I have a LT155 john Deere lawn tractor with a kohler CV15s_41562. While mowing, it started making a loud banging noise and I killed it. Now it won't start. I installed a new ignition module but still won't start. I pulled the valve cover and the intake valve lifter and push rod does not move. The exhaust moves fine but not the intake. What do you think, a collapsed lifter or a stuck lifter possibly. Engine turns over easily with no loud noises.