The cam followers don't tend to 'fall out' normally. If it was over reved and the valves 'floated' then it's possible that it could have popped out. Another possibility is a collapsed lifter. The best way to reinstall this is to remove the cam and place the rocker back into position. If it's a follower and not a roller 'follower' then they 'must' be put back on the cam lobe that they originally work with otherwise the wear pattern will be different.
Once you get the cam out you can lift the lifters one at a time and measure their height against each other. They should all be about the same height. If that one lifter is shorter then you have identified your problem. Replace all the lifters just to be safe.
Once you get the cam out you can lift the lifters one at a time and measure their height against each other. They should all be about the same height. If that one lifter is shorter then you have identified your problem. Replace all the lifters just to be safe.