How can I get my Niagara Modules back?

I wanted to examine the “blueprints” (I don’t know the actual term) for the Niagara modules supplied with the game engine to learn how they are written. Then I could write my own in Scratchpad. I double-clicked on one in the System Overview window, looked over the node program in the editor that came up and when I was done, I clicked on the “x” in the corner of the box. I 'm sure now this was a big mistake. Since then, if I try to look at the inner workings of any other module, it is blank. I get a Niagara Log with the comment " Script compile status unknown." I cannot find a control to restore this function. I actually went to the Epic site, removed UE4.25 from my computer and re-installed it. This made no difference. The capability is also missing from my copy of 4.26. Is there anything I can do to restore this functionality?

You closed the node graph window:

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