Extremely loud backfire/rockers off kilter

Hey fellas, I as working on my 400 the other day installing a new adjustable vac advance. I put everything back together plug wires etc. I then tried to start the car and it wouldn't fire up. I kept trying to get it to fire for about 10 seconds at a time. Finally I rechecked my plug wires and everything checked out. I then attempted to start the car when a huge single backfire almost took my hearing out....then the car started as normal. What could cause this????Incorrect valve settings???fuel from trying to start the car???
today when I removed the valve cover on the driver side when I noticed my #7 rockers were off the spring and the pushrods were nowwhere to be found. I took off my roller rockers and guideplates and looked into the lifter valley. I found my lifters and pushrods still intact but they were out of there sockets. I put the lifters back in there sockets and pushed down on them with a pushrod to see if the still had spring (hydraulic) one did not which I think is bad Correct? So my question is.....Did the backfiring cause this situation with the #7 cylinder and lifters? Is that hydraulic lifter bad if there is no spring in it? Any help would be appreciated.

dont know what caused the problem, but sounds like probably the engine was flooded, and your ignition finally kicked hard enough to blow that fuel, which could have pushed the rocker off the valve... just put her back together, adjust your valves again since the rocker came off... and then see if its all good

I don't know if this is your trouble or not. But one time after I worked on my distributor I started my car and boom it sometimes backfired like a gun going off. The problem turned out to be a wire grounding out on the metal inside the distributor.