Looks like the retaining clip popped. You should go back and look for the clip. Some folks have taken the lifters apart, cleaned them up and put them back in the motor (imperative to put each lifter back in the hole it came out of). I myself have never done that so others here might be able to weigh in and I would imagine some here may have an old lifter to scrounge a new clip from. What I have done is replace lifters with new lifters on an old cam, you have to use cam lube, EOS or some other cam breakin oil, and broke the new lifters in just like a new cam. If you go this route, try to find new hydraulic lifters that are either nos or rnos like 20 or better years old. The new one that are sold have a somewhat checkered history with failure and collapsing. A friend of mine has a machine shop and rebuilt a 54 power glide 235 with new lifters for a customer of his. Had to pull that motor three time because of the lifters collapsing. I gave him a set of nos gm and no more problems.
