When you alter a file in use by UE4 in a way that makes it invalid for the engine’s purposes and Accept the changes, it removes the (no longer valid) file from the Assets list but does not delete it from the folder.
Unfortunately, when you replace it with a Valid copy it does not restore it to the asset window/list.
Sometimes you can save it under a new name then rename it. Otherwise you have to manually delete it and reload the project to get it to recognize that file again.