There are many possibilities that might be causing this issue, Please take a look at this article Lightroom: External hard drives not displayed in my Library any longer. | Photoshop Family Customer Community
Let us know if that helps.
What do you mean by "see"? Exactly where are you looking in Lightroom? Please explain. What do you expect to see? What do you see? Describe in detail.
Note: Lightroom is not a file browser like Mac Finder. It isn't supposed to "see" anything like your file browser does, it only shows files that have been imported into Lightroom.