I'm very new to Illustrator CC. I did something to make layer 2 invisible and I don't know how to undo it.
A picture is worth a thousand words..
Not entirely sure how you would do so accidentally (there's no default keyboard shortcut as far as I know), but you've set the group's (not the layer itself) opacity to 0.
Simple fix... set the opacity back to "100".
Tip: You can see in the layers panel if a layer has any appearance attributes other than a basic fill/stroke (such as a modified opacity) by the filled dot alongside that layer.