You may not be out of the woods yet. Was the pushrod going up and down the same amount as the others? You could still have a worn cam or collapsed lifter. If you don't want to start the engine with the valve cover off, you can pull the plugs and crank it over with the starter while watching things.
The only way the pushrod can come out of the rocker is:
1- Adjustment was way, WAY too loose
2- Cam wiped resulting in the lifter/pushrod dropping enough to come out of the rocker
3- Lifter collapsed resulting in the pushrod dropping enough to come out of the rocker
Unless the lifter is moving the same amount AND at the same distance from the top of the head, you'll need to pull the side covers and look at the lifter. A visual inspection will tell if it's collapsed. They are cheap.
I guess it's possible the adjustment nut backed off enough to let the pushrod out but I've never heard of that happening....not to say it can't.
